Change masterproc to mainproc

[ekluzek]

Use of master in SE and IT is outdated with a preference for main. This is actually a better technical description, since here the mainproc actually does more work than the other processors, and is NOT just a coordination role. The mainproc sends most of the logging output and some tasks after gathering everything from the other processors.

There is an effort in SE and IT to have more inclusive language. One such terminology list is here:

https://docs.google.com/spreadsheets/d/1ZktyU8_foZGWQHzzTc2sinyvtmhAn96maCbN8JwLn2c

See the CTSM issue regarding this here: ESCOMP/CTSM#1127

@mvertens has this implemented in #74