Update the observation date modeling that has been changed during the

[lmichel]

During the MANGO REC process, it has been suggested to simplify the representation of the observation dates (mango:EpochPostion.obsDate).
The mango:ObsDate complex type has been replaced with a simple attribute where the representation is given by its dmtype (<ATTRIBUTE dmrole="mango:EpochPosition.obsDate" dmtype="year" value="2000.0" \>).

This change requires the SkyCoord builder to be updated as well as the annotation builder.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.15%. Comparing base (8316767) to head (f89b913).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #726      +/-   ##
==========================================
+ Coverage   84.10%   84.15%   +0.04%     
==========================================
  Files          79       79              
  Lines        8608     8603       -5     
==========================================
  Hits         7240     7240              
+ Misses       1368     1363       -5     

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