New high level API and general rework

[mnottoli]

The old high level Fortran APIs were not properly done, and the C ones were missing. This pull request addresses these two problems and brings in some additional content to make future development easier.

This is a list of the content:

• New model nonspecific APIs in Fortran and C. There is a set of functions to set up, fill the guesses, solve the linear systems and compute energies and forces. To circumvent interface problems, all the electrostatic properties are now stored in a container (ddx_electrostatics_type)
• New monolithic solver routine in Fortran and C. A new version of ddsolve has been implemented, this solves the linear systems and computes the energy and (if requested) the forces. Now, it uses the electrostatic properties container and optional arguments to avoid unnecessary allocations. A similar version is also implemented in C.
• Consistent naming. All the APIs have consistent names in Python, C and Fortran.
• Moved the deprecated content (only used by the tests) to the module ddx_legacy.
• Added a new ddinit which is a wrapper around allocate_model and uses optional arguments. This makes it possible to add future extensions without breaking retro-compatibility with software that it is not aware of the new arguments (e.g. in case of new models / new functionality that requires some settings).

[mnottoli]

This is the last of the three pull requests. @mfherbst, I will send you further information.

All the tests (Python, Fortran, Psi4) are passing.

This builds up on #148. To close this, we can do the following. After #148 gets merged I would suggest creating a new local branch, cherry-picking all the relevant commit from this PR (from a444609 on) and then rebasing on the new main. After that, a git push --force can be used to overwrite the content of this PR, with the rebased PR. This avoids creating a new PR.

[mnottoli]

Rebase done, ready to be merged.