[newchem-cpp] cppify interp_table_utils.{h->hpp}

[mabruzzo]

To be reviewed after #428 is merged

This is a really tiny PR. I'm introducing it because it will be very helpful in finishing #447. It did 3 minor things:

1. converted interp_table_utils.h -> interp_table_utils.hpp
2. I added the ability to use delete[] for cleaning up a gr_interp_grid_props instance in anticipation of a change in [newchem-cpp] introduce GrainMetalInjectPathways type #447 (the change probably could have waited until then, but it was small enough to do it now).¹
3. I relocated the function for initializing empty gr_interp_grid_props and gr_interp_grid out of initialize_chemistry.cpp into interp_table_utils.hpp

Footnotes

1. I'm hesitant to go through and convert change every malloc/free to new[]/delete[] involving a gr_interp_grid_props or a gr_interp_grid in this PR because I can't remember if I introduced any more instances in pending PRs. Honestly, I don't think I did, but if I'm wrong, debugging a mismatched malloc/delete[] would be extremely annoying. ↩