create unit test for radial return function

[rob-hamill]

create simple unit test that does the following:

• checks expected output for a defined input
• checks approx. duration of function for known input and output (so future updates don't make it hang excessively etc)
• optionally checks shapes, conventions of variables throughout (may be covered by validation / checks in the function itself)

Use some pseudo random sampling (like LHS sampling) to down sample our input
Also generate the results from the algorithm to compare.
Can save these as npy files