bug fixes:
- weighted_mean
- deal with nans in median
- dataset: convert scalar to DimArray's scalar
- added __and__ and __or__
- added Polygon and Mask classes to dimarray.geo.region, with vectorized
  `contains` (mpl_toolkits dependency)
  method
- renamed parameter axes to dims in lib.transform.interp2d_mpl
added automatic transpose to DimArray.put if values to put is a DimArray
fix a bug in indexing in broadcast_array mode