can't specify 2d vs 3d chunks from open_mdsdatastore

[rabernat]

read_mds now has the ability to specify 2D vs 3D chunks: https://xmitgcm.readthedocs.io/en/latest/utils.html#xmitgcm.utils.read_mds

However, this option is not accessible from open_mdsdataset: https://xmitgcm.readthedocs.io/en/latest/usage.html#open-mdsdataset

We should allow the user to specify chunks='2D' or chunks='3D' when calling open_mdsdataset. I'm not sure the current chunks keyword is actually used by anyone (or that it actually works as described), so we shouldn't worry about overriding its previous meaning.

[raphaeldussin]

in #98 I propose to rename chunks='ND' by chunking_method='ND' not to be confused with the actual size of the chunk. Also CS needs a special treatment of the chunking_method