Small fixes/updates for file-io object (#392)

Tag name (required for release branches):
Originator(s): peverwhee

Description (include the issue title, and the keyword ['closes',
'fixes', 'resolves'] followed by the issue number):
- Fix bug for integer and real scalar variables (missing allocate
statements)
- Add variable name to error message for clarity

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
- add allocate statements for scalar pointers
- Add varname to error message via get_pio_errmsg()

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

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:

Author: peverwhee