PIO reader variable subsetting functionality

[nusbaume]

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Originator(s): nusbaume

Description (include the issue title, and the keyword ['closes', 'fixes', 'resolves'] followed by the issue number):

Adds subsetting functionality to the "pio_reader" implementation of the File I/O NetCDF reader class. This includes both the ability to subset a variable via "start" and "count" optional variables, and a series of automatic Fortran unit tests to make sure the code is working as expected.

However, please note that the unit test coverage is not complete (i.e. there are certain array rank/variable type combinations that are not covered), but hopefully this is good enough to at least support what is needed for the current set of CAM-SIMA target applications.

Closes #406

Describe any changes made to build system: N/A

Describe any changes made to the namelist: N/A

List any changes to the defaults for the input datasets (e.g. boundary datasets): N/A

List all files eliminated and why: N/A

List all files added and what they do: N/A

List all existing files that have been modified, and describe the changes:
(Helpful git command: git diff --name-status development...<your_branch_name>)

M src/physics/utils/pio_reader.F90
M test/unit/fortran/src/pio_reader/test_pio_reader.pf

• Add variable subsetting functionality to the PIO reader object.

If there are new failures (compared to the test/existing-test-failures.txt file),
have them OK'd by the gatekeeper, note them here, and add them to the file.
If there are baseline differences, include the test and the reason for the
diff. What is the nature of the change? Roundoff?

derecho/intel/aux_sima: All pass excluding known test failures (the multitape test).

derecho/gnu/aux_sima: ALL PASS

If this changes climate describe any run(s) done to evaluate the new
climate in enough detail that it(they) could be reproduced:

CAM-SIMA date used for the baseline comparison tests if different than latest:

[jimmielin]

Thanks @nusbaume making this really important functionality possible! I have two suggested changes which hopefully are straightforward to fix.

[jimmielin]

Thanks @nusbaume!

[peverwhee]

Looks good, and I confirmed it works as expected for the solar irradiance data in CAM-SIMA