add Dask support for zonal_stats and zonal_crosstab (#502)

* include zone with id 0

* nodata

* apply: nodata param

* noqa

* flake8 fixes

* docs

* nodata

* refactor

* allow floating zones

* stats: added dask support

* stats: updated examples

* zone column dtype

* zone column dtype

* update examples in docs

* crosstab 2d

* crosstab 2d: numpy case

* crosstab 2d: dask case

* flake8 fixes

* crosstab 3d: int layer

* crosstab 3d: dask case

* refactored

* refactored

* flake8 fixes

* allow floating zones agg

* nodata zones

* nodata zones

* update tests

* nodata_values

* docs

* docs

* stats: nodata_values

* stats: update tests

* condense tests

* flake8 fixes

Author: thuydotm