Search multiple corpora

[lukavdplas]

A much requested feature is the ability to search multiple corpora simultaneously.

It should be relatively modest task to allow users to search any selection of corpora with limited functionality, but some functions are a lot more involved. Many features such as filtering and visualisations become troublesome to interpret when corpora don't share the same fields. A potential solution is to implement 'core fields' (#993).

Another solution would be to focus on cross-corpus search in specific subsets that have similar definitions, such as newspaper corpora. This is less flexible to the user but would probably satisfy the most common use cases.