Version number slightly broken

[phackstock]

As pointed out by @danielhuppmann, there are certain cases in which:

import nomenclature
nomenclature.__version__

prints out 0.0.0 or some other wrong number.
It's also 0.0.0 in readthedocs https://nomenclature-iamc.readthedocs.io/en/stable/.

Update

After some detective work with @meksor (thanks a lot), I think we've figured it out.
The issue with readthedocs should be resolved with #329.
@danielhuppmann the issue that you've reported where your local version number is 0.0.0. seems to be related to the same issue, i.e. not having the poetry dynamic versioning plugin installed. If you've installed nomenclature from GitHub using pip install instead of poetry install, I'd be curious how you did that since it should not work anymore.
That being said, the installation instructions from GitHub are outdated in the README and in the docs.
I think we should remove this option altogether from the README and the docs and just suggest installing from pypi.
We'd keep the instructions on how to install from GitHub only in the DEVELOPING.rst.

Finally, the way nomenclature.__version__ currently works is that it's using importlib.metadata.version. This reads as the name suggests the metadata file in the site-packages. This is only set when you run poetry install. However, running poetry install in a poetry repository emulates the behavior of running the -e flag for pip install. This means that all current local changes will be active but nomenclature.__version__ will show the version number from the last time it was installed.
This is potentially confusing and can be remedied by using poetry version to get the version number instead.
I could implement it this way.