Add stricter type requirements

[matze]

Data is currently identified as an n-dimensional array of float arrays. However, certain filters require the input to contain semantically more specialized data (e.g. ifft input should be complex values). Passing more type information along the pipeline allows to catch subtle bugs.

There are two ways to implement this: 1) adding fields to the internal structure and add more API to query this or 2) using the meta data infrastructure. Open for discussion.