Provide option to specify additional directories storing run-specific data files.

[arrieta]

This enhancement request asks for providing an option to specify additional directories storing run-specific data files.

Currently, EMTG reads data contained in universe_folder. For example, it loads all ephemerides contained in universe_folder/ephemeris_files.

It is desirable to have the capability to provide additional directories that store ephemerides or similar data.

A concrete case we have in mind is:

• User has a directory containing commonly-used spice ephemerides and ancillary data (e.g., de430.bsp, naif0012.tls, pck00010.tpc, and so on). These are used in nearly every run
• User has a separate directory that contains problem-specific data (e.g., ephemerides for a given asteroid that is only used in one specific EMTG run)
• User is able to specify the first directory, the one containing commonly-used data, via the usual options.universe_folder variable; this option is seldom changed (if at all)
• User is able to specify the second directory, the one containing problem-specific data, via a new option, possibly called options.case_data_folder