Add obsid info

[matteobachetti]

This is a trivial addition: when present, the OBS_ID header keyword is read and added to the meta information of the FitsTimeseriesReader (and the EventList that will be created from it)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.05%. Comparing base (b2e6e1a) to head (9faeebf).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #863      +/-   ##
==========================================
+ Coverage   94.21%   96.05%   +1.83%     
==========================================
  Files          48       48              
  Lines        9687     9689       +2     
==========================================
+ Hits         9127     9307     +180     
+ Misses        560      382     -178     

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[eleonorav89]

No comments from me, and thank you for your work