Extend test to the reduction to the pole transformation

[santisoler]

Description of the desired feature:

Currently the reduction to the pole filter and transformation are being tested with a re-implementation of the filter in the frequency domain, and a comparison of the results with an RTP grid created with Oasis Montaj.

It would be nice if we can add an extra test that uses a synthetic model (like a single dipole) to produce a grid with arbitrary inclination, declination (both for the inducing field and for the source itself), and a grid with the same source in the pole. Then we can run our RTP function on the first grid and compare it with the latter.

Are you willing to help implement and maintain this feature?

Sure! But I'd love to see anyone else to tackle this!