this fixes several issues:
- allows users to download terrain data at home with no GPS lock
- allows for a TERRAIN_CHECK without GPS lock
- retries opening of files every 5 seconds to allow for remount of sd
card after boot
this fixes a problem where two different locations could both be
mapped to the same disk block in the terrain/*.DAT files. That meant
that pre-filled terrain on the microSD card would sometimes require a
download in flight. It also means that a RTL with loss of GCS could
sometimes fly through a region with no terrain data available
Other changes in this patch:
- allow for a 2cm discrepancy in the lat/lon of the grid
corners. This is needed to allow for slightly different floating
point rounding in tools that pre-generate terrain data to load on
the microSD
- added TERRAIN_OPTIONS parameter to allow the user to disable
attempts to download new terrain data. This is mostly useful for
testing to validate a terrain generator
This commit changes the way libraries headers are included in source files:
- If the header is in the same directory the source belongs to, so the
notation '#include ""' is used with the path relative to the directory
containing the source.
- If the header is outside the directory containing the source, then we use
the notation '#include <>' with the path relative to libraries folder.
Some of the advantages of such approach:
- Only one search path for libraries headers.
- OSs like Windows may have a better lookup time.
if we run out of terrain data then extrapolate using the last
available terrain height at the AHRS position. This can be used to
cope with GCS outages over long distances where the terrain data isn't
preloaded
