Eliminate the use of temporary files in the vx_config library

[JohnHalleyGotway]

Describe the Enhancement

Please see #2690 which is the parent issue derived from the dtcenter/METplus#2364 discussion.

The vx_config library is one major source of the use of temp files in MET. They are used in two ways:

1. Each config file read() function reads the input line-by-line. Each line is passed through a utility function to recursively interpret any environment variables referenced. The output is written to a temp file and subsequently read back in.
2. The logic for parsing thresholds is also handled in by the config file code. The read_string() function is called to do so. It writes the threshold string to be parsed to a temp file prior to calling the read() function to handle it... which uses its own temp file! So at least 2 temp files are used for each threshold string!

This issue is to leverage the std::stringstream class to eliminate the use of temporary files in the vx_config library. While the I/O difference may not be obvious when writing to the local /tmp directory, it will vastly improve performance on the lustre file system used by many HPC's, including WCOSS2 and jet.

Time Estimate

1 day

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[JohnHalleyGotway]

Note that PR #2692 merged a fix into the develop branch for this. However, I'm going to keep this issue open since it sounds like we want to provide the same fix in the main_v11.1 branch.