feat: support Mw/kcat models from GECKO 3.0

[carrascomj]

Description

As risen by @edkerk, the matlab toolbox GECKO 3.0 has changed the specification of stoichiometric coefficients for enzymes (see here). The coefficients which were previously $\frac1 {k_{cat}}$ has been changed to be $\frac {M_w} {k_{cat}}$.

Geckopy should remain compatible with GECKO, so it should support this coefficient fashion.

Implementation

This PR achieves (shallow) interoperability at the document level; i. e., the writing and reading function of SBML now accept a flag to read SBML models with the GECKO3 encoding. The default remains the same (i.e., legacy encoding) and the inner workings of geckopy are still in $\frac1 {k_{cat}}$.

Reading and writing with the $\frac {M_w} {k_{cat}}$ will raise an error if no $M_w$ is found in a protein. For reading, this relies on the annotation attribute, using "mw" as the field (to follow the name used in GECKO3 YAML, see this example). A test was implemented for reading and writing with this format.

I had to remove support for python 3.6 since Github Actions does not support it for ubuntu-latest (and it is deprecated anyways). python3.10 was not added to CI but it should in the future (see #11).

[carrascomj]

(@phantomas1234 I cannot add you as reviewer)

I believe this is the bare minimum of compatibility, let me know what you think.

[carrascomj]

(Merged after offline conversation)