Clean-up of test documentation and code
Added test function for dm_shape.
Fixed function call
Add expectation for compute_dm_shape

The only expectation is that it should be a float. The actual value is
not checked.
Implemented the computation of the DM shape.
Give floats as position to compute the shape.
Fixed plotting.

Note that this files are strictly for prototyping.
Merge pull request #6 from forcaeluz/dm_shape

Dm shape computation.