style guide / python or petsc4py style

[lmoresi]

We have a real mix of python / C++ styles in different parts of the code. I think it's because we sometimes inherit the style choices from petsc4py which tries to follow PETSc.

In the constitutive models we have a traditional python lower_case_underscore choice for arguments to classes and functions but in most of the solvers we have lowerCaseUpperCase patterns.

Doesn't matter much, but can we choose one or the other ?

L

[lmoresi]

@julesghub - what we need is an actual style guide that we can follow.

This should include the use of the black formatter, and should also have python / cython in its sights. See the pure-python/cython branch discussion as well.