[newchem-cpp] cleanup lookup_cool_rate_1d part 1

[mabruzzo]

This shouldn't be reviewed until after #412 and #413 are merged

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As part of this PR, I

1. factored all of the logic for interpolation collisional rates out of lookup_cool_rate_1d into a helper function
2. introduced gr_opaque_storage::used_kcol_rate_indices:
   • during initialization of gr_opaque_storage::kcol_rate_tables, we now only initialize the "ordinary" reaction rates that we use
   • we store the indices of each of these rates initialized rates within gr_opaque_storage::used_kcol_rate_indices. The length of this list is tracked by gr_opaque_storage::n_kcol_rate_indices
3. refactored the interpolation of every "ordinary reaction rate" so that instead of explicitly naming each reaction rate, we now iterate over entries recorded within gr_opaque_storage::used_kcol_rate_indices
4. slightly refactored the logic for computing the density-dependent k13 rates to let us avoid heap allocating the scratch space

[mabruzzo]

@brittonsmith, this should be ready to go