Added a skeleton greens function
Restructured code to work with arrays
Few changes to style of greens_function()
Added a skeleton for xyz_grid
Transpose secondary source positions

This way, arrays of coordinates are easily iterable.

Also: use np.squeeze to make sure input arrays are one-dimensional.
Updated greens function
Convert input to float64
Merge branch 'mono' of github.com:sfstoolbox/sfs-python into mono

* 'mono' of github.com:sfstoolbox/sfs-python:
  Convert input to float64
  Updated greens function
  Transpose secondary source positions
  Added a skeleton for xyz_grid
  Few changes to style of greens_function()
  Restructured code to work with arrays
  Added a skeleton greens function