Add Zhu dust+molecule+atom table opacity variant and example file.

+ 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).

singularity-opac/photons/opac_photons.hpp

@@ -17,6 +17,7 @@
 #define SINGULARITY_OPAC_PHOTONS_OPAC_PHOTONS_
 
 #include <singularity-opac/photons/epbremsstrahlung_opacity_photons.hpp>
+#include <singularity-opac/photons/zhu_grey_table_opacity_photons.hpp>
 #include <singularity-opac/photons/gray_opacity_photons.hpp>
 #include <singularity-opac/photons/non_cgs_photons.hpp>
 #include <singularity-opac/photons/photon_variant.hpp>
@@ -33,9 +34,10 @@ using Gray = GrayOpacity<PhysicalConstantsCGS>;
 using PowerLawScaleFree = PowerLawOpacity<PhysicalConstantsUnity>;
 using PowerLaw = PowerLawOpacity<PhysicalConstantsCGS>;
 using EPBremss = EPBremsstrahlungOpacity<PhysicalConstantsCGS>;
+using ZhuTable = ZhuTableOpacity<PhysicalConstantsCGS>;
 
 using Opacity = impl::Variant<ScaleFree, Gray, PowerLawScaleFree, PowerLaw,
-                              EPBremss, NonCGSUnits<Gray>,
+                              EPBremss, ZhuTable, NonCGSUnits<Gray>,
                               NonCGSUnits<PowerLaw>, NonCGSUnits<EPBremss>>;
 
 } // namespace photons