Consistent handling of paths

[Peter9192]

The ewatercycle user interface uses string formats to get path inputs. It would be helpful if we internally default to using standard Path representations as well. #82 implements a pathparser that converts input strings to pathlib objects. It would be nice use this throughout the package.

It might also be nice to add some custom __str__ methods so that the user never gets to see the internal representation.

[sverhoeven]

By fixing #113 we no longer leak paths to user with the public API.

Converting strings to Path objects is now duplicated in each model. We can make generic function in ewatercycle.util to perform this conversion so expanduser() and resolve() are called always. Something like

def make_absolute(p: str) -> Path:
  return Path(p).expanduser().resolve()

[Peter9192]

Yep, like that 👍

Don't we still leak paths when the user tries to inspect the ParameterSet or Forcing objects?

[Peter9192]

I made a start with something like that in #107

[Peter9192]

ewatercycle.parameter_sets.default.make_absolute() can be reused in ewatercycle.forcing.default. Maybe move to util.