Masked array support

FINAM seems to be well prepared for masked arrays. Some changes:

• removed Grid.mask feature again, all masking is done with masked arrays (reverts !258 (merged))
• removed all compressing related methods from the grid classes and moved them to tools as convenience routines

Added some convenience routines for masked arrays in data.tools:

• is_masked_array: check if given data is a masked array
• has_masked_values: check if given data is a masked array and has some values masked
• filled: create a filled copy of the data if it is a masked array
• to_compressed: return only unmasked raveled data
• from_compressed: create full (masked-)array from raveled data
• check_data_covers_domain: meant to check init data in connect phase if data covers the desired domain that is specified by a mask on the given grid

all routines respect quantified and un-quantified data.

Fixes #59 (closed)