Updates in to the latest spec changes

[tschaub]

This updates a number of things to deal with changes in the CQL2 spec. Most of these changes came in opengeospatial/ogcapi-features#831

Breaking changes include:

• Previously, all "user defined" functions were in an object like this {"function": {"name": "custom"}}. Now, these are collapsed into the existing built-in "op" expressions. So {"op": "custom", args: []} is the equivalent (args are required).

• Previously, case insensitive and accent insensitive expressions were in {"casei": "Foo"} and {"accenti": "bǎr"} type expressions. Now these are collapsed into the existing built-in "op" expressions. So {"op": "casei", "args": ["Foo"]} and {"op": "accenti", "args": ["bǎr"]}.

Another change here is that interval arrays are string literals instead of date/timestamp objects. This was either my misreading the spec previously or another change.

I've added validation for all JSON output using the latest CQL2 JSON Schema.

This should be more stable now since voting on the spec is underway.

Fixes #62.

[bjgiraudon]

Thank you so much for that update!

That was blazingly fast, nice job @tschaub!