Issue 501 os to pathlib

[finozzifa]

Closes # (if applicable).

Issue #501

Changes proposed in this Pull Request

The PR aims at replacing the os module with the pathlib module

Checklist

• I consent to the release of this PR's code under the AGPLv3 license and non-code contributions under CC0-1.0 and CC-BY-4.0.
• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Newly introduced dependencies are added to envs/environment.yaml and doc/requirements.txt.
• Changes in configuration options are added in all of config.default.yaml and config.tutorial.yaml.
• Add a test config or line additions to test/ (note tests are changing the config.tutorial.yaml)
• Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• A note for the release notes doc/release_notes.rst is amended in the format of previous release notes, including reference to the requested PR.

[finozzifa]

mmm tests were ok when I executed them locally. I need to figure out what's the matter. Apologies for the failing pipelines.

[finozzifa]

Hi all,

please find below a recap of the changes I performed in this PR.

I tried my best to include all of your suggestions and recommendations. If I missed some, I apologize and I will include them as soon as possible.

### Recap of the changes

1. The os module has been replaced with the pathlib module. There are still a few os-module-calls in build_natura_raster.py (this is because there is no pathlib alternative for os.walk) and in the _helpers.py

2. As mentioned before, I have introduced an abstraction of the path methods. Such methods are available in the _helpers.py script. In fact, whenever I see lines of code that repeat themselfs several times, I try to create methods that implement such lines of code and that can be separately unit-tested. In this was I can ensure consistency and make future changes easier. Instead of tracing throughout the code each invocation of (say) os.path.basename(), I just need to modify once the method where such line is implemented. I believe that in this way, code reviews, code merges and further code expansions will be easier, as we can rely on soundproof methods that we have unit-tested.

3. I have introduced dedicated unit tests for the path methods I coded in the _helpers.py. As of now, these unit tests are NOT executed as part of the CI/CD pipelines.

### Possible spin-offs

1. It would nice to push these modifications a bit further, so that the dependency on the os and pathlib modules is explicit just in the _helpers.py script.

2. It would nice to include the execution of the unit tests in the CI/CD pipeline, as already done for the framework repository

I thank you in advance for your comments in reviewing this PR :)

Fabrizio