Add a configurable option for importing CompareImages

Describe the Enhancement

When running test/run_all_nco.sh or test/pythest on WCOSS2, the code errors out with:

ModuleNotFoundError: No module named 'imutils'

These failures were not present in the beta5 installation, and 'imutils' is not allowed on WCOSS2.

It appears this situation came about from this PR Feature 461 make compare images configurable #467 where the line "from metcalcpy.compare_images import CompareImages" was uncommented from approximately 20 files. CompareImages in METcalcpy imports 'imutils'. It looks like the PR intended to make the use of CompareImages configurable by an environment variable used in conftest.py. But, conftest.py is not what is used to test on WCOSS2, instead run_all_nco.sh is used. However, when running pytest the 'imutils' errors are still present.

The code was commented out again in this PR 461 revert compare images changes #472.

We need a configurable option so that the CompareImages isn't imported on WCOSS2, but could be elsewhere if desired.

Time Estimate

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[x] METplus, MET, METdataio, METviewer, METexpress, METcalcpy, METplotpy

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dtcenter / METplotpy