Add more thorough FOV tests

[dahlend]

Added

• Added more complete testing for light delay computations in the various FOV checks.

Fixed

• Field of View checks for SPK checks was not returning the correct light delayed time,
  it was returning the position/velocity at the observed position, but the time was
  the instantaneous time.

Tests now verify that:

• two-body checks are accurate to ~150km
• N-body accurate to 150m
• SPK checks accurate to 150 micron