Don't validate ReservoirRfcParameters paths if reservoir forecasts are disabled

[aaraney]

fixes #799

troute.config -- 0.0.3

Changes

• If reservoir_rfc_forecasts is disabled, the reservoir_rfc_forecasts_time_series_path directory will not be validated to exist in strict mode (fixes Configuration validation fails in strict_mode when files that aren't needed at runtime are specified #799).

@shorvath-noaa, are there other fields that I use this same kind of pattern on? The idea being, that in strict mode, we only need to validate the paths that troute will use.