Merge pull request #17 from FRidh/bands

Move constants into module namespace
Turbulence: Change indexing for calculation of field.

Generating a field of size (x,y) resulted in a shape where the
dimensions were ordered as (y,x). This patch changes the ordering from
cartesian indexing to matrix indexing.

A very simple test is included as well to test whether object creation
and field() execution goes without issues.
Initial PSTD code