update warnings module and reduce pgi optimization

[apcraig]

Update the warnings module to make it more thread-safe. Reduce the pgi optimization consistent with recent CICE changes.

• Developer(s): tcraig

• Are the code changes bit for bit, different at roundoff level, or more substantial? bit-for-bit except for pgi.

• Is the documentation being updated with this PR? (Y/N) N
  If not, does the documentation need to be updated separately at a later time? (Y/N) N

• Other Relevant Details:

Am getting intermittent recursive I/O runtime errors in the warnings package in CICE. Not clear why that is but suspect it might be threads competing in the write phase of the warnings package. This might or might not fix it, but it does improve the implementation in any case.

Reduced the pgi optimization. O2 was giving reproducibility issues in CICE, that was reduced to O, so we're doing the same here.

Icepack test results are here, https://github.com/CICE-Consortium/Test-Results/wiki/icepack_by_hash_forks, hash d4d68f1. CICE tests are being carried out now.

[apcraig]

I tested this in CICE and things are fine. Results are https://github.com/CICE-Consortium/Test-Results/wiki/cice_by_hash_forks hash 3b78bd. The cray results are not bit-for-bit due to an earlier change in Icepack, adding "dhi = c0, dhs = c0" in icepack_therm_vertical.F90. Not sure why this only affected cray and not other compilers. I have confirmed this. I have tested on 5 compilers and all the failures are understood and not a problem as far as I can tell.