Inconsistencies in catalog specification

[m-mohr]

I went through the catalog specification, i.e. the readme (README.md), recommendations (static-recommendations.md) and JSON Schema (json-schema/catalog.json). There are several inconsistencies and problems with it:

• Fields are not clearly defined (see also Better define what should go in 'provider' field #78, License field #133, Homepage field in the catalog #137, Format field in the catalog #138)
• The recommendations mention a "Product". What is this about? It is not mentioned in the Readme or JSON Schema.
• I think a "properties" property should be mentioned for the catalogs as @matthewhanson mentioned in the chat yesterday that the EO extension (and others) could be used in the catalog, but it's not clear where to put them.
• The JSON schema needs to be updated, it is not up-to-date with recent developments of the rest of the spec. Examples: Style of links (Array vs. Object for lists of links #134 / Update catalog.json to new 'links' style #127), License (License field #133), datetime vs. startDate / endDate (partly covered by API date/time filtering #130), ...
• There is a property "TODO" in the JSON schema of the catalog.

[m-mohr]

Should be fixed with #239 .

[m-mohr]

JSON schema is getting updated in #250.