Add dask-related tests to test suite

[BSchilperoort]

All routines should work lazily with Dask. This should be tested.

[slevang]

In #49 I added a parametrize over chunking schemes to the core regrid tests, so we know they at least work with dask. We should still add a check for laziness though. dask is also not a core dependency of xarray-regrid right now, so ideally we should parametrize the tests with and without dask usage.