datamodel: create base structure for datamodel

[ppanero]

Create a base structure for the DataCite based datamodel. This aligns with the records datamodel inside cookiecutter-invenio-instance (previously self-contained in cookiecutter-invenio-datamodel).

It contains:

• JSON Schemas
• Elasticsearch mappings
• Loaders
• Serializers
• Deserializers

NOTE: There's still a lot of discussion to be had on what should be internal and what metadata, etc. This just aims to have a clear code structure of the datamodel and a starting point to add more fields.

Closes #6

[ppanero]

NOTE*: I think both commits can be squashed in one

[fenekku]

This is great for setting up the groundwork on which to build the different schema fields.

[ppanero]

@fenekku addressed all your comments :) Thanks a lot!

Just reminds the keyword. I would create an issue and leave as it is for now, to be consistent with cookiecutter (which was the point of the issue). And have a further discussion on the keywords/subjetc(s)/etc. and change as a result in the future. What do you think?

[fenekku]

Looking good. The last 2 things for me are:

• title sorting (let's take care of it now) + suggesting: what is the use case for title auto-completion (when are we doing this?)?
• the date / date-time usage for publication_date : why different?

I will approve, so once you resolve them you can merge.