Document the config settings which support template tag substitution

[JohnHalleyGotway]

Describe the Task

The #2152 discussion was about where the template tags, such as {valid} can appear in the METplus configuration file settings.

While it is sort of mentioned in the section about observation data under this section, it should really be clearly stated at the top of the Directory and Filename Template Info section.

This issue is to update the METplus User Documentation to clarify this.

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2 hours?

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[georgemccabe]

Changes in PR #2187 include a fix to allow filename template tags in _DIR variables, not just _TEMPLATE variables. This was the main source of confusion as to where the filename template tags can be used. Closing this issue because that update is sufficient for resolving the root issue.