Refactor field info parsing to read once then substitute time info for each run time

[georgemccabe]

PR #868 mentions to consider adding this functionality. We learned that SeriesAnalysis does not currently support using filename template tags in field info variables (FCST/OBS_VAR_*). This is needed for a use case @j-opatz is working developing, so this seemed like a good time to go ahead and implement this.

Describe the Enhancement

Read field info variables on initialization, skipping the step to substitute string values for each run time. Store the field info templates to be substituted for each run time.

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<1 day

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4.1.0

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