Fix Point-Stat and Grid-Stat logic for processing U/V winds with python embedding

[JohnHalleyGotway]

Describe the Problem

This issue arose while investigating dtcenter/METplus#1931. The specific issue for this discussion topic can be seen on this line of grid_stat_conf_info.cc. When using python embedding to read U and V wind data, the calls to req_level_name() return empty ConcatStrings. However, since empty ConcatStrings are NOT considered to be equal, the code never finds U/V matches, and never writes VL1L2 output.

Could modify the is_uv_match() logic to handle the fact that these requested level strings are always empty. This enables the python U/V matching logic to succeed. However, we would NOT be able distinguish between multiple levels being provided via python embedding.

We could also support the specification of level string in python embedding. That'd enable us to distinguish between different levels.

When checking for U/V matches, look for MULTIPLE matches and print a warning message if that happens.

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