Add type key to properties (simple)

[CasperWA]

This addresses #269, but does not close it. That will be up to #270.

The type key will hold the value of the outermost type of the property it describes, i.e., for species, which is described as a "list of dictionaries", the type will be "list".

The value of type MUST be one of the OPTIMADE data types described here in the specification.

[giovannipizzi]

Looks useful to me - only doubt I have now is about the units being just a string, especially when you e.g. have composite units: is speed "m/s" or "m s^-1" something else? I'll move this to a different issue

[ml-evs]

I'm happy with this as is, but do you think there should be a discussion of the type field in the section about provider-specific types? i.e. "Providers MUST/SHOULD provide the type of custom fields at the info/entry endpoint", or some such? Otherwise I don't see what value is gained from this PR alone for clients trying to display custom fields (as probably no-one will implement this field).

[CasperWA]

I'm happy with this as is, but do you think there should be a discussion of the type field in the section about provider-specific types? i.e. "Providers MUST/SHOULD provide the type of custom fields at the info/entry endpoint", or some such? Otherwise I don't see what value is gained from this PR alone for clients trying to display custom fields (as probably no-one will implement this field).

That would be great, but considering so many of the properties are already not a MUST, I think it's hard to justify this point.

However, we could always add a RECOMMENDED?

[rartino]

When we later upgrade this field to be a more complete type description; will we have to consider this a backwards-breaking change (i.e., new major version number) or a minor version?

How about we write in this text to that if the value isn't a simple OPTIMADE Datatype, the client should ignore the field?

[rartino]

Alright, I'll make my suggestion a concrete edit.

[CasperWA]

In the latest commit I've added the following:

• A database-provider-specific property MUST be listed in its respective entry listing /info endpoint.
• Future-proofing the type field.
• New :type: rst role to be used when writing OPTIMADE data types.