block+PyBlock: proxy method for create_data_array
block::create_da: make dtype&shape param optional

shape is actually not really optional because we have
no support for 'data' yet. But it will be in a few commits.
[test] add block::create_da with dtype=None test
block::create_da: add optional data parameter

And infer dtype and shape from data param if not given.
[test] block::create_da: test data parameter
block::create_da: add documentation
PyBlock+docstrings: remove doc for _create_da

_create_data_array is no internal and needs no doc; the
doc was outdated anyway.
Merge pull request #72 from gicmo/block_create_da

Block create_create_data enhancements

Very nice. But aren't you in vacation exploring french savoir-vivre :-)