pp_config does not honor --caseroot option

[jedwards4b]

the pp_config script assumes that it is in the postprocess directory before it checks the value of the --caseroot option, this results in an error:
pp_config ERROR: env_postprocess.xml does not exist in this directory.

[bertinia]

--caseroot should be --pp_caseroot in order to support both stand-alone and existing CESM caseroot instances.