Add derivatives to EOP when they are missing in the files
Two sets of EOP data have derivatives: polhody (xp, yp, as derivatives will be added in early November 2023 by IERS in several products and are already present in EOP C04 since February 2023) and dUT1 (because LOD is linked to dUT1 derivative). In some cases, the derivatives are present in the files Orekit loads, in some cases they are missing.
If for example one loads EOP C04 data from before Febrary 2023 and data after November 2023, the polhody data will be inconsistent.
Bulletin A also miss LOD data. In the regular txt version of the files, this is taken care of during parsing, and some finite difference algorithm is used to add the missing data. This is not done in the xml or csv versions of the file.
It would be better and more consistent to add missing derivatives to points that miss them during loading, as a post-processing step before the EOP history is returned. This way, all data will always have all derivatives and interpolation will be more consistent. This is a follow-up of issue #1238 (closed) as it will permit proper Hermite interpolation throughout history for these parameters.