Added array casting to Elementwise and Aggregation like operations.

Fixes #82.
I did play with having an ``as_any_array`` function at one point, though I got scared off when I started thinking about other things that we might like to be able to handle with this (NetCDF.Variables etc.). What are your thoughts on having a *simple* ``as_any_array`` (which takes either a ``biggus.Array`` or a ``np.ndarray`` and returns a ``biggus.Array``).