Fix many build warnings

[rhoneyager-tomorrow]

Description

Hey guys, I'm building jedi-bundle develop for the first time in a while, and am seeing lots of compiler warnings with icx / ifort. These compilers are being generous by passing much of this with warnings instead of raising errors, and I'd recommend tightening up the CI checks to catch these.

C++:

• The oops CMake logic has been using classic Intel compiler flags where it should be using appropriate options for the new Clang-based compiler. That's because it is matching "Intel" against compiler ID, but "IntelLLVM" also matches.
• You can't declare a runtime variable-length array on the stack. E.g. int field_vid[vars.size()] invokes undefined behavior. Prefer std::vector<int> field_vid(vars.size()).
• Return values can't be references to temporary objects.
• Under higher logging levels (e.g. -Wall and -warn all, there are many overloaded virtual function warnings. Essentially, base class functions are being masked (not overridden) with different parameters in derived classes, and that's messy.

Fortran:

• Many functions are being referenced without a declaration (e.g. without extern and outside of a module).

Impact

At the very least, this fixes failures in test_util_signal_trap_fpe_div_by_zero and test_l95_getkf_cpp that I observed. Conversely, it exposes transient bugs in test_assimilation_etkf and test_qg_increment that should be diagnosed.

Checklist

• I have performed a self-review of my own code
  • Yes, and I tested locally.
• I have made corresponding changes to the documentation
  • No changes here.
• I have run the unit tests before creating the PR

[fmahebert]

Looks good to me. Thanks @rhoneyager-tomorrow!

(sorry for slow review, I seem to never have gotten the notification about this one)

[srherbener]

Thanks for fixing these!

[fmahebert]

I've opened a PR in JCSDA-internal from your branch, @rhoneyager-tomorrow.

[fmahebert]

The PR in jcsda-internal was merged, so these changes should propagate to public. I'll close this PR now.

Thanks @rhoneyager-tomorrow for contributing these warning fixes!