Add mask to Info object

Closes #124 (closed) #118 (closed) #111 (closed) #113 (closed)

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 can be initialized with dim or data_shape now
  • NoGrid.data_shape can have -1 entries for variable size dimensions
  • if only dim given to NoGrid, all entries in data_shape will be -1

Fixes

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