Indices for decomp_cpools_vr in SoilBiogeochemDecompCascadeMIMICSMod scope issue

### Brief summary of bug

In MIMICS there are private [indices](https://github.com/ESCOMP/CTSM/blob/2b288500a2c9d16e6306e1afaf9a0ab0686a2d7f/src/soilbiogeochem/SoilBiogeochemDecompCascadeMIMICSMod.F90#L51-L54) with the same purpose as the global ones in clm_varpar:
```fortran
integer, private :: i_phys_som ! index of physically protected Soil Organic Matter (SOM)
integer, private :: i_chem_som ! index of chemically protected SOM
integer, private :: i_avl_som ! index of available (aka active) SOM
integer, private :: i_str_lit ! index of structural litter pool
```

This means that the global ones are created they will never get assigned proper values in `init_decompcascade_mimicsplus()`.

### General bug information

**CTSM version you are using:** [output of `git describe`]

latest master

**Does this bug cause significantly incorrect results in the model's science?** [Yes / No]

Not yet but will give a runtime error if f.e. `i_phys_som` is used outside the SoilBiogeochemDecompCascadeMIMICSMod like `i_met_lit`.

**Configurations affected:** [Fill this in if known.]

Any with soil_decomp_method='MIMICSWieder2015'