Introduce the possibility to modify package update/create schema

[seitenbau-govdata]

We want to deactivate the email validation for the fields author_email and maintainer_email, because we would not like to reject the import of a dataset if the email address is invalid.

The pull request introduces the possibility to modify/overwrite the update/create package schema used in the harvest process updating/creating datasets. This can be done by implementing the new interface methods update_package_schema_for_create and update_package_schema_for_update.

[amercader]

This sounds useful, I might steal it for other harvesters! My only concern is that we will now have two ways in which harvesters can be extended, inheriting from this base class and overriding the _get_create/update_package_schema methods or implementing the IDCATRDFHarvester interface (which implements after/before_update/create).

Perhaps for consistency we can add new methods to that interface?

[seitenbau-govdata]

Good point, @amercader ! We will try to change it and make use of the IDCATRDFHarvester interface.

[seitenbau-govdata]

@amercader I have changed the pull request to using interface methods. Please review again. :-)