DataAxis bugfix + value2index takes ndarray

Fixed a bug in DataAxis._get_name() when the axes_manager was not set.
Also, DataAxis.value2index() now accepts ndarrays as input.
DataAxis.value2index was not using the rounding argument when value was a numpy array.
Add tests.
Automatic style corrections courtesy of autopep8.
Update value2index docstring
Merge pull request #1 from francisco-dlp/data-axis_bugfix

Data axis bugfix
Merge pull request #276 from vidartf/data-axis_bugfix

DataAxis bugfix + value2index takes ndarray