Refactor `Equivalent`

This issue is to refactor the `Equivalent` solver to make efficient use of the `Borefield` class and the refactored `heat_transfer` module, and to simplify the code.

This could be an opportunity to eliminate the `_EquivalentBorehole` and `_EquivalentBorefield` classes. Much of their methods already have close equivalents in the `Borehole` and `Borefield` classes.