Fix summarize_cprnc_diffs to work with new test scripts

[billsacks]

(1) Get rid of -in_rundir option: now cprnc output is always in the
run directory

(2) Just look at cprnc output files with 'base' in their name

(3) Rework usage message to be correct with new test scripts

Test suite: None
Test baseline: N/A
Test namelist changes: N/A
Test status: N/A

This script is not covered by any system or unit tests. I tested it
manually by running it on a test suite and comparing the output with
the previous version.

Fixes: None

Code review: None

[billsacks]

cc'ing @sholly

[billsacks]

In principle, RMS results could now be obtained from the TestStatus.log files rather than this tool finding them again in the cprnc.out files. However, I didn't do that because:

(a) It was already written to parse the cprnc.out files.

(b) If I remember correctly, the TestStatus.log files contain any RMS entries, from either baseline comparisons or the test itself (e.g., from the exact restart test). I don't think there is an easy way to separate these two sets of entries, at least not without doing some serious parsing of the TestStatus.log file. It can be confusing if these two types of differences are grouped together, so I thought it best to just parse the cprnc.out files directly, so I could restrict this parsing to the base.cprnc.out files.