Make angles computation optional for KLM

[mraspaud]

The KLM data contains angular relationship that could be used instead of computing the angles with pyorbital.

[mraspaud]

A little more information: the angular_relationships item in each KLM scanline provides sun and sat zenith, plus an azimuth difference. POD scanlines have a solar_zenith_angles.
The missing angles could be computed from the earth_location (available for both KLM and POD) and spacecraft_altitude_above_reference_ellipsoid (available for KLM, not for POD (but doesn't change much in principle)).

My suggestion is to use the available angles and allow computation of others from the provided parameters when the TLEs are missing (either no files or gaps) and issue a warning.

[mraspaud]

Related: #65