Change all paths in pyaerocom from strings to Path-like

[lewisblake]

Is your feature request related to a problem? Please describe.
strings are used all over. A better choice would be to use e.g., Path from pathlib. Then pyaerocom can be used in a VRE, we can read data directly from Thredds. The code would then be agnostic to if it's reading a real path or a URL.

Describe the solution you would like to see
Above

• Would you be able to work on this solution yourself?
• How can the Pyaerocom development team assist you?

Additional context
Add any other context or screenshots about the feature request here.