`'equivalent'` solver for fields of inclined boreholes

[MassimoCimmino]

The implementation of an 'equivalent' solver for inclined boreholes poses theoretical challenges that fall outside of issue #50, dedicated to the adaptation and implementation of known solutions. This new issue will necessitate additional work for validation and optimization of the code, if the implementation can be achieved.

I am including this into the v2.2 milestone but might consider postponing to a later release.