INS: use an accessor method on the state

Publically exposing the Nav structure was bad design.
INS: Remove old 16 state version

Not used and previous change broke it.  It will be in history
if needed.
INS: get rid of unused Nav structure
INS: make functions that should be static

This code is quite old and was committed before our standards were
so high.
GCS Controller: add periodic timer to send GCSReceiver

This is a quick solution to problem 508 where the GCS Controller does
not successfully set GCSReceiver frequently enough to remove a failsafe.
When using GCSReceiver mode when you move the stick to a valid position
it will stay connected.
GCS Controller: only transmit updates when enabled

When the GCS Control checkbox is enabled then GCSReceiver outputs
will start occuring periodically.  With this patch control works
but still ManualControlCommand is set to read only which blocks
it.

Work on #508.
GCS Control: move changing ManualControlCommand meta to the gadget

The original architecture is first of all problematic.  Way too much
logic is in the widget (UI) instead of the gadget (controller).  This
was a problem for enabling GCSReceiver control since it was clobbering
ManualControlCommand meta to read only which blocked the FC from reading
the "Connected" flag.

Fixes #508
Fix Q_DECL_EXPORT bug that caused Windows breakage.
Merge pull request #543 from kubark42/fix-telemetry-scheduler

Fix Q_DECL_EXPORT bug that caused Windows breakage.
Merge pull request #533 from peabody124/hitl_fix

GCS Controller / HITL fix
Merge pull request #531 from peabody124/ins_get_state

INS cleanups
When starting HiTL, GCS set the home location, but did not identify it
as set=true. This patch fixes the behavior, as well as a minor variable
change to improve clarity.
Introduced new UAVOs in preparation for path planning refactor.
Changed STANDARD_AIR_SEA_LEVEL_PRESSURE from KPa to Pa.
Removed fixed-wing settings aimed specifically at CC. This was no longer
necessary.
Added MIN, MAX, and sign macros to misc_math.h. The sign macro returns -1
for values less than 0, and 1 for values greater than or equal to 0. 0 is
specifically not handled different from strictly positive numbers.
Added path manager module. The path manager's function in the navigation
hierarchy is to pass the appropriate path segment to the path follower, and
monitor the path follower's progress.

Several new functions were introduced into misc_math. These functions
calculate certain geometries which are useful for dubins paths.

Lastly, three sample paths were provided. 1) A return home, 2) a position
hold, and 3) an example path that can be used as a base for further
path programming.
Refactored path planner. The path planner's role in navigation is to take
user-defined waypoints and provide a path that connects them together.

The path planner is supplied with two algorithms, "direct" and "direct-
with-filleting".

"Direct" goes directly to the waypoint, no matter how severe the
angle between two consecutive waypoints.

"Direct-with-filleting" adds fillets
to the direction changes, in order to provide smoother, more realistic
paths for dubins cart vehicle classes, i.e. vehicles that have a minimum
turn radius.
Added circular moudulus for radians.
Refactored fixed-wing path follower to use math from known good version.