Clean up low-zoom layer

- Move preserved railways and monorails from low-zoom layer #roads-low-zoom
- Remove funicular rule that never gets hit
Update coding guidelines

Update coding guidelines after discussion with Andy:
- Code order
- Notation of multiple attachments
Layout
Changes layer order

The layer order used to be as follows.

(bridges)                               B

I classified all layers between and including the bridge-layers and the tunnels-layer with one of the following classifications:

B    Bridge level.
R/B  Above roads but below bridges.
R    Road level.
T/R  Above tunnels but below roads.
T    Tunnel level.

I grouped all of the layers within one class together, without changing the order within the classes. For now, I ignored the buildings-layers, due to the difficulties with ordering buildings, highways and highway-areas described in #172.

This change causes a couple of changes in the rendering, which mainly occur in rare cases.
- Casings of turning circles are now drawn above foot/bike/track tunnels (they were already drawn above other tunnels), and above cliffs and barriers (just like other roads are drawn above cliffs and barriers).
- Landuse-overlays (i.e., military and national park), castlewalls, and citywalls are now drawn above foot/bike/track tunnels (they were already drawn above other tunnels).
- Ferry-routes are now drawn below roads (highways, tracks, turning-circles, and highway-areas).
- Railways on level 12 (due to the #roads-low-zoom layer), aerialways, and waterway-bridges are now drawn above access and directions markers of roads (they were already drawn above the roads themselves).
Changes layer order

The layer order used to be as follows.

(bridges)                           B
direction-pre-bridges.directions    R
access-pre-bridges.access           R
waterway-bridges                    R/B
roads-low-zoom                      R/B
aerialways                          R/B
ferry-routes                        T/R
turning-circle-fill                 R
roads-fill                          R
tracks-notunnel-nobridge            R
buildings                           ???
buildings-lz                        ???
highway-area-fill                   R
roads-casing                        R
highway-area-casing                 R
area-barriers.barriers              T/R
cliffs                              T/R
line-barriers.barriers              T/R
tracks-tunnels                      T
footbikecycle-tunnels               T
turning-circle-casing               R
landuse-overlay                     T/R
castlewalls-poly.castlewalls        T/R
castlewalls.castlewalls             T/R
citywalls                           T/R
tunnels                             T

I classified all layers between and including the bridge-layers and the tunnels-layer with one of the following classifications:

B    Bridge level.
R/B  Above roads but below bridges.
R    Road level.
T/R  Above tunnels but below roads.
T    Tunnel level.

I grouped all of the layers within one class together, without changing the order within the classes. For now, I ignored the buildings-layers, due to the difficulties with ordering buildings, highways and highway-areas described in #172.

This change causes a couple of changes in the rendering, which mainly occur in rare cases.
- Casings of turning circles are now drawn above foot/bike/track tunnels (they were already drawn above other tunnels), and above cliffs and barriers (just like other roads are drawn above cliffs and barriers).
- Landuse-overlays (i.e., military and national park), castlewalls, and citywalls are now drawn above foot/bike/track tunnels (they were already drawn above other tunnels).
- Ferry-routes are now drawn below roads (highways, tracks, turning-circles, and highway-areas).
- Railways on level 12 (due to the #roads-low-zoom layer), aerialways, and waterway-bridges are now drawn above access and directions markers of roads (they were already drawn above the roads themselves).
Merge branch 'roadsv2.0' of https://github.com/math1985/openstreetmap-carto into roadsv2.0
Merge branch 'documentation' of https://github.com/math1985/openstreetmap-carto into math1985-documentation
Soften the language slightly for this rule.
Merge branch 'roadsv1.1' of https://github.com/math1985/openstreetmap-carto into math1985-roads
Re-instate funiculars at low zooms.

The original omission in the SQL query is the bug, and it makes sense
to treat them similarly to other railway types, and to have them
appearing not after their stations when zooming in.
Merge branch 'roadsv2.0' of https://github.com/math1985/openstreetmap-carto into math1985-roadsv2.0

Conflicts:
	project.mml
Merge remote-tracking branch 'official/master'

Conflicts:
	roads.mss