Added nebular emission from Diffused Ionized Gas (DIG)

[prerakgarg07]

Update log:

• Nebular emission from DIG can now be added (Still under active development)

• New parameters added:

  1. add_DIG_neb: Flag to turn on/off nebular emission from DIG.
  2. DIG_nh: Gas hydrogen density for calculating DIG nebular emission in units of cm^-3.
  3. DIG_min_factor: Sets the minimum energy dumped that has to be included for DIG calculations

• Dump cell info now saves cell widths and cell centers in centimeters instead of code units