implement MgII (2796 & 2804) line transfer
Add 4 new modules:
- module_MgII_2796_model.f90
- module_MgII_2804_model.f90
- module_gas_composition_MgII.f90
- module_gas_composition_MgII_dust.f90
and modify
- module_constants.f90
- module_ramses.f90 => add subroutine ramses_get_T_nMgII_cgs
- Makefile
The main validation is that we can reproduce Fig. 4 in Prochaska et al. (2011) with their fiducial wind model (except that their density is two times higher and J.X.Prochaska agreed this is a typo in their paper).