Refactor heat_transfer module

[MassimoCimmino]

This follows #210.

Functions in the heat_transfer module have long lists of borehole parameters (e.g. H, D, r_b). With the implementation of the Borefield class, this can be simplified to only take Boreholeobjects, lists of boreholes or Borefield objects as input.

This is backward-compatible with regards to the finite_line_source function but not to the other functions in the modules.