Unify usage of CRS object

[Jaapel]

Kind of request

Changing existing functionality

Enhancement Description

Currently, crs in the code can be a str, int or pyproj.CRS object. HydroMT should be flexible in parsing, but strict in internal code. This is not yet the case. We should use the same CRS object in all internals.

Use case

General Maintainance effort and readability of the code will benefit from these changes.

Additional Context

No response