Centralize `NUM_SUBVOLS` for HACC simulations

[aphearin]

Currently, at the top of each diffmah fitter script, there is a global variable that defines how many subvolumes into which the HACC simulation has been partitioned (e.g., NUM_SUBVOLS_LASTJOURNEY for Last Journey). This same variable is also used by diffsky, and so is redefined elsewhere. This variable should be added to the returns of the get_diffsky_info_from_hacc_sim function to avoid inconsistencies.