Draft: Add mask to Info object

Closes #124 #118 #111 #113

Enhancements

• added Mask enum with two options:
  • Mask.FLEX for flexible masking
  • Mask.NONE to explicitly use plain numpy arrays
• added mask attribute and init-arg to Info : can be a Mask value or a valid mask for numpy.ma.MaskedArray
• data.tools.prepare now applies masks to data if set in Info object
• ARegridding now has a out_mask arg
• RegridNearest and RegridLinear now support explicitly masked data (input doesn't have Mask.FLEX)
• adapters now have an in_info property

Changes

• Info now has properties for grid, time and mask
• all init-args of Info are now optional
• Info.accepts has changed signature: renamed ignore_none to incoming_donwstream
• Info.accepts now only checks: grid, mask and units (other meta data can differ)
• Grid.to_/from_canonical now allows additional dimensions
• data_shape now a property of GridBase (NoGrid has an empty data_shape)

Fixes

• cells for structured grids in 3D are now created correctly (no negative Volume in VTK/ESMF)