INS: allow a default mag value for indoor mode

Since we do not perform navigation in this mode, requiring
a GPS to get the HomeLocation or manually setting it is not
necessary. Also, forcing people to set it in the map is
bad because that saves by default - and then they might
get in the field with a very incorrect home location.
PID: do not set integral to zero when Ki is zero

This was done in the PID library but is not appropriate there. The
altitude hold code uses the integral term to apply the throttle
bias. If the integral is wanted to be zero, we should set the ILim
to zero.
AltitudeHold: use antiwindup code for the integral component

This ends up not really making a practical difference as the
throttle doesn't hit the limits for long.
VtolFollower: make sure to preload integral for throttle correctly

Since this is the integral for the down command, need to use
the negative of the throttle.
pid: update copyright header
Merge pull request #1154 from peabody124/vtol_fixes

Altitude Hold tweaks