Allow mapping dataset variables to extra metadata

[forman]

There is often a need for extra metadata associated with a dataset's (geophysical) variable which cannot be (easily) derived from the variable's data arrays or netCDF/CF attributes. Such metadata could be

• ancillary variables: names and roles of variables associated with a given variable, such as masks, uncertainty
• default visualisation type, default chart type;
• display value range, colour bar for image visualisation. These should be aligned with the values used in the CCI Visualisation Tool.

Such variable to extra metadata mappings would ideally be stored in a JSON/YAML configuration file (e.g. in ~/.cate/mappings.yaml) so users can easily edit and adapt.

[JanisGailis]

This is at least partly solved in CF-compliant datasets:
http://cfconventions.org/cf-conventions/v1.6.0/cf-conventions.html#ancillary-data

We already use this convention in outliers detection when creating a mask.