Allow cosmology to be specified for mock spatial calculations

[manodeep]

For the calculations that involve converting from redshift (or cz) to co-moving distance, the conversion depends on the cosmology. Therefore, the relevant codes DDrppi_mocks and vpf_mocks should either let the user pass in the co-moving distances directly or let the user specify the cosmology. Currently, cosmology has to be implemented directly in the code itself -- in utils/cosmology_params.c under the function init_cosmology. This change will also remove the need for the globals declared/defined under utils/cosmology_params.[ch].

Both solutions should be implemented. The first one (user does the calculation) is easy: just check the flag is_comoving_distin struct config_options defined in utils/defs.h.

The second one (cosmology specified at runtime) will need a little more attention. There is no machinery at the command-line executables to handle cosmology. And I really would like to keep the python API identical to the command-line.

[manodeep]

is_comoving_dist is now implemented. May be in a later version, I will think about adding in the arbitrary cosmology support at the command-line level.