Major updates

Creates a module-wide way to set defaults for :

So, for example, to set the backend to use the dot product implementation, call:

import xagg as xa
xa.set_options(impl='dot_product')

Alternatively, use a with block for individual chunks of code, here using an example with silent:

with xa.set_options(silent=True):
     wm = xa.pixel_overlaps(ds,gdf)

which silences all status updates.

Bug fixes

Closes #55 ; silent=True now should suppress all status updates to standard out.