Add explicit API for mixed layer CAPE/CIN calculation

[akrherz]

Previous issues #16 and #438 are relevant to this, but unless I am missing something obvious, I am not seeing a direct means to calculate mixed layer CAPE/CIN. I was expecting to see some API that had a depth= option available and a function name like mixed_layer_cape_cin.

Thanks for the consideration.

[zbruick]

I've started on this in #1145...it's not complete and I'd appreciate feedback on it, as user controls are going to be important for that function. Also to have benchmarks to test against to make sure we mix correctly and then calculate the profile and CAPE/CIN.