Added warning to Fermi detector angle calculation code, for when time returned is different from time requested

[aringlis]

In the fermi module of sunkit-instruments, the code get_detector_sun_angles_for_time returns the angle with respect to the Sun of each Fermi/GBM detector. This is done by reading in a Fermi spacecraft weekly file.

However, these files contain gaps due to the South Atlantic Anomaly. If the time requested falls in one of these gaps, the code will return the detector angles for the next available time. This can be several minutes different from the time requested. Currently, the code does not provide any indication that this has happened.

This PR adds a warning to the user if the time returned by the code is more than 1 minute different from the time requested (1 minute is the nominal cadence of the spacecraft weekly file), and explains that this is likely due to a South Atlantic Anomaly encounter.

[nabobalis]

Thanks again @aringlis