Boolean indexing with multi-dimensional key arrays

Originally from https://github.com/pydata/xarray/issues/974

For boolean indexing:

• da[key] where key is a boolean labelled array (with any number of dimensions) is made equivalent to da.where(key.reindex_like(ds), drop=True). This matches the existing behavior if key is a 1D boolean array. For multi-dimensional arrays, even though the result is now multi-dimensional, this coupled with automatic skipping of NaNs means that da[key].mean() gives the same result as in NumPy.
• da[key] = value where key is a boolean labelled array can be made equivalent to da = da.where(*align(key.reindex_like(da), value.reindex_like(da))) (that is, the three argument form of where).
• da[key_0, ..., key_n] where all of key_i are boolean arrays gets handled in the usual way. It is an IndexingError to supply multiple labelled keys if any of them are not already aligned with as the corresponding index coordinates (and share the same dimension name). If they want alignment, we suggest users simply write da[key_0 & ... & key_n].

No implementation files, tests, or entry points are named. Start by tracing the existing boolean-indexing behavior and comparing it with the three cases described here; done means the proposed labeled-array indexing and assignment semantics work consistently, including alignment errors for multiple keys.