+ Add ZhuTableOpacity photon class with Rosseland-Planck databox.
+ Assume log-log rho-T for DataBox interpolation.
+ Add ZhuTable variant to opac_photons.hpp.
+ Add unit test for ZhuTable variant.

Notes:
- See: Zhaohuan Zhu et al (2021).
- The next step is to convert the Zhu ASCII into HDF5 format(s).

+ Use filesystem header to check extention in ctor.
+ Refactor old ASCII parser to a private function (called if .txt).
+ Add ZhuTable::Save method and use to add HDF5 file.
+ Update unit test to parse HDF5 data instead.

Thanks to Jonah Miller for the suggestion to use the Save method to
create the HDF5 file.

Co-authored-by: Jonah Miller <[email protected]>

Co-authored-by: Jonah Miller <[email protected]>