Update naming convention for ephemeris files

[FredSchuller]

After discussion with ESA, the agreement was that the ancillary files with ephemeris and pointing info shoul be named:
solo_ANC_stix-aux-ephemeris_..._V...fits

To do: update pieces of code here and there to make them compatible with the new names.

[FredSchuller]

To be decided: should the code be backward compatible (i.e. able to read both "aux" files with previous naming and new naming)?

[nicHoch]

@FredSchuller please note the new name:

i4Ds/STIXCore#396

and as we still do not have approval from ESA please wait until i ping you in that issue.

To your main question: for a easy transition it would be very nice if it is backward compatible. But this totally depends on you how much effort this is and if the code still stays maintainable for longer time.

if it is not backward compatible we have to plan the release more carefully

[samaloney]

Closed in #228