add memory usage variables for use on derecho

[jedwards4b]

Description of changes

Adds variables MEM_PER_TASK and MAX_MEM_PER_NODE for greater control of memory options on derecho - it can also be used elsewhere but initially only on derecho, requires cime and ccs_config changes.

Specific notes

Contributors other than yourself, if any:

CMEPS Issues Fixed (include github issue #):

Are changes expected to change answers? (specify if bfb, different at roundoff, more substantial)

Any User Interface Changes (namelist or namelist defaults changes)?

Testing performed

Please describe the tests along with the target model and machine(s) If possible, please also added hashes that were used in the testing