Full support for reading/writing masked arrays. (#6)

- Gave up masking for scipy matrices because it seems pretty scratchy.
- DelayedMask returns True for is_masked.
- Improved storage optimization to use the masking'ness of the input array.
  This avoids block-by-block decisions on masking status.
- Pass along buffer sizes when optimizing storage.
- Added some more tests just in case.

src/dolomite_matrix/DelayedMask.py

@@ -134,3 +134,8 @@ def is_sparse_DelayedMask(x: DelayedMask):
     """See :py:meth:`~delayedarray.is_sparse.is_sparse`."""
     return delayedarray.is_sparse(x._seed)
 
+
+@delayedarray.is_masked.register
+def is_masked_DelayedMask(x: DelayedMask):
+    """See :py:meth:`~delayedarray.is_masked.is_masked`."""
+    return True