WIP: Apply transforms on download

Allow the user to specify transforms to be applied between materialization and format conversion. This can be used to do transforms like sampling (see #132 (closed)) or aggregation (#98).

I'm not sure where the transforms themselves should live. The apiserver seems inadequate, but so is lib_materialize.

• Add parameter to the download endpoints
• Do transform in datamart_core.materialize.get_dataset()
• Implement some transforms
• Write tests
• Frontend?