Refactor: Instead of str paths us pathlib.Path

[markus-hinsche]

Motivation: The recommended way of handling paths in Python3 are pathlib.Path.

Status Quo: Currently we use both:

• str-like path
• pathlib.Path

Solution: Refactor larger parts of the code to use pathlib.Path instead of str-like paths. Parts of the code could be tackled

• src/common/evaluation
• depthmap_toolkit
• src/models/