ENH: Array.get_axis_num method

This provides a standard way to get axis numbers for an xray array, i.e.,
`axis = array.get_axis_num(dim)` instead of
`axis = array.dimensions.index(dim)`.

The main advantage is that it gives a sensible error message.
Merge pull request #94 from akleeman/xarray-axis

ENH: Array.get_axis_num method