Vertically resolved C variables dimensioned levsoi, while N variables dimensioned levdcmp

### Brief summary of bug

The inconsistency described in the title should be corrected by using the levdcmp dimension for both C and N variables. Additionally we should make levdcmp = levsoi (does not include bedrock) rather than levgrnd (includes bedrock layers). For consistency we should use levdcmp rather than levgrnd when writing these variables to restart.

Extra credit:
We may wish to list all the "lev" dimensions and their uses to assess whether we may consolidate some of them.

### General bug information

**CTSM version you are using:** ctsm5.1.dev106-7-ga5b74c35b

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

**Configurations affected:** bgc

### Details of bug

Although this doesn't cause errors, it complicates the use of history and restart files, for example in the case of the Newton-Krylov spin-up tools. For this reason, I will open a PR to correct.