BUG FIX: Serialize collapsed coordinates by datatype, not units

This fixes a bug introduced by #847, which is encountered when attempting to
collapse along a dimension coordinate with unknown units. In such a case, the
coordinate value is converted into a string, which fails for dimension
coordinates because dimension coordinates must be numeric.

In contrast, this new patch chooses how to serialize a collapsed coordinate by
its datatype instead of its units. String point values are joined by '|',
whereas all other point values (which are presumed to be numeric) are replaced
by the mean.
Correction to MagicNumber __init__
Merge pull request #959 from bjlittle/file_element

Correction to MagicNumber __init__
MAINT: Extended the STASH_TRANS table
ENH: Nearest neighbour regrid adds crs.

This adds the crs to the target grid if possible (is an iris coordinate
system).  There are also various documentation tweaks as part of this.
MAINT: Review actions taken
Merge pull request #894 from cpelley/regrid_doc_clarification

ENH: Nearest neighbour regrid adds crs.
Merge pull request #973 from cpelley/new_stash2lbfc

MAINT: Extended the STASH_TRANS table
BLD: Scipy travis update to 0.12.0
Add ppa syntax change
Merge pull request #960 from cpelley/latest_scipy

BLD: Updading travis scipy package.
BLD: Update travis numpy v1.7.2
Merge pull request #975 from cpelley/travis_numpy_update

BLD: Updated numpy to 1.7.2
Minor update to top-level readme
1.7.0-dev
Merge pull request #977 from bblay/v1.7.0-dev

1.7.0-dev
Merge pull request #978 from esc24/readme_update

Minor update to top-level readme
aggregate with string coords

test
Merge pull request #980 from marqh/flex_agg_sq

aggregate with string coords
DOC: Updated whatsnew to 1.6.0