Field distortion and transverse diffusion map

[ftoschi]

What is the problem / what does the code in this PR do

This PR includes the possibility to include field distortion coming from a initial-to-final position map (typically coming from COMSOL simulations). This is possible by setting the field_distortion_model to comsol, while by default it is set to none: in case this is specified, the map stored in field_distortion_comsol_map will be used to map the radial initial position of a given event to a final distorted position.

In addition, this PR enables the possibility of using transverse diffusion map(s), assuming that these are divided into radial and azimuthal components as it would come from a 2D-axisymmetric simulation.

WFSim needs you:

• Please add a test for this PR, as a bare minimum, make sure it's covered in coveralls!
  • Will provide
• Can you add a docsting to all your functions?

Pay attention:

• Due to databases being needed for testing, making a PR from your own fork will typically NOT run the tests. If you then merge master might break