Yuhsuan/895 hypercube types

[YuHsuan-Hwang]

This PR is for the second part of #895:

1. Extended the types of hypercube:
   apply a general check of the FITS stokes axis;
   convert CARTA::PolarizationType to casacore::Stokes;
   apply error handling for stokesCoordinate().
2. Fixed issues when reading stokes info from the header:
   allow CTYPE to be "Stokes" or "stokes" for CASA and FITS images;
   get stokes indices using CRPIX, CRVAL, and CDELT to correctly read headers with CRPIX != 1;
   convert FITS index to CARTA::PolarizationType.

requires frontend branch: yuhsuan/1606_hypercube_types

[confluence]

I would prefer to avoid putting anything in Util unnecessarily -- it should only be required if a function is used in multiple files which do not and should not depend on each other.

GetCasaStokesType is only used by StokesFilesConnector, so I think that it should be defined in that header (and if we do decide to put it in Util, for consistency it should go in Util/Casacore).

ConvertFitsStokesValue could be defined in the FileLoader header, since StokesFilesConnector imports it.

Apart from that, I think this all looks good.

[YuHsuan-Hwang]

@confluence I moved the functions to StokesFilesConnector.h and FileLoader.h. Please check, thanks!

[kswang1029]

this works well 👍