SSP_D_Ld4.f09_g17.I1850Clm50BgcCrop.cheyenne_intel.clm-ciso_rtmColdSSP fails with MIMICS

### Brief summary of bug

Error message:
`micr1n is required on an initialization datasetERROR in SoilBiogeochemNitrogenStateType.F90 at line 463`

### General bug information

**CTSM version you are using:** ctsm5.1.dev074-3-ga16cc85ac

**Does this bug cause significantly incorrect results in the model's science?** Test just fails.

**Configurations affected:** soil_decomp_method = 'MIMICSWieder2015'

### Details of bug

Make micr1n optional in initial files? Or need MIMICS-specific initial file because it contains this variable?