Add an alternate entry for variables that may have more than one standard_name associated with them

[wrongkindofdoctor]

What problem will this feature solve?
Some variables have multiple potential standard_name attributes (e.g. "rainfall_flux" vs "precipitation_flux"), and the query needs to able to check for both.
Describe the solution you'd like

• add an optional "alternates" entry to the fieldlist variables following the POD settings file. This information is parsed during the fieldlist query and can be appended to the varlist.alternates attribute
  Describe alternatives you've considered
• variable-specific retry in the preprocessor--far from ideal as alternate list grows
• precip_to_rainfall pp function similar to rate_to_flux conversion: better, but also not ideal to have a function for each variable conversion
  Additional context