Modify imports in top level scripts to match new location of scdmesh.py.
Format consistency.
tests -> unit_tests for custom source.F90
read_header() in source_gamma.F90 now skips comment lines at start of 'gammas' file.
letter -> letters for list storing parameters.
Added comment lines to test_header.txt; Tests pass fine.
Changes to keep 'git status' output cleaner.