Add height dependence option to the ZM convection entrainment rate (dmpdz)

### What is the feature/what would you like to discuss?

Adding an option to make ZM deep convection test parcel entrainment rate have a simple linear dependence on height. An extra dmpdz_ut variable is added and a height transition region between the default lower troposphere value (still dmpdz) and this ut value. The default will have dmpdz=dmpdz_ut, effectively unchanged from the current constant everywhere value of dmpdz value.

### Is there anyone in particular you want to be part of this conversation?

@JulioTBacmeister @adamrher

### Will this change (regression test) answers?

Yes

### Will you be implementing this enhancement yourself?

Yes