Merge CuPy and Dask tests and datasets

[pentschev]

To allow for easy testing of CuPy and Dask during this phase, a copy of the existing relevant Dask tests was created under https://github.com/dask/dask-glm/pull/75. This incurs in a lot of duplication that can later be merged into a single implementation to test both backends. The same happens for the datasets module, which was duplicated for the time being.