Add support for $comment in property definitions

[rartino]

JSON schema has in draft 7 added support to put comments in schemas via $comment: http://json-schema.org/understanding-json-schema/reference/generic.html#comments

When working on our property definitions in #445 I independently realized it would be useful to insert comments in the property definitions. So we should add this to the subset of JSON Schema that we allow.

[merkys]

What would be the target audience of comments? Maybe we can just put them to description?

[rartino]

It would be for comments that are relevant to property definition "developers" rather than "end users". Another difference is that you can put it on any level in a nested definition to comment e.g. the motivation behind setting a limit like "maximum" to something. I stumbled on the desire to insert a comment when I realized I wanted somewhere to explain why common/element.yaml doesn't have an x-optimade-property (it is because it isn't itself part of the OPTIMADE standard properties, it is just used as a target for '$ref' when we compose other standard properties.)

On the other hand - writing this up makes me realize that we might want to specifically have a way to strip certain comments when we inline '$ref' references, since the comment I mention above would be nonsensical after inlining the definition.

[merkys]

OK, this makes sense.

[ml-evs]

Closed by #457 (I think) or a related PR