Add sanity check to error out when both is_u_wind and is_v_wind are set to true

[JohnHalleyGotway]

Describe the Enhancement

This issue arose in the dtcenter/METplus#1970 discussion. The user defined both is_u_wind = TRUE; and is_v_wind = TRUE; inside the fcst and obs dictionaries rather than specifying them separately for each field. This certainly is an easy thing to do and warrants that more error checking be added to MET.

In VarInfo::set_dict(...), add a check so that if both is_u_wind and is_v_wind are set, we error out and write a useful error message.

Time Estimate

2 hours.

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