Introduction of PackageWrapper & ResultWrapper

[BvB93]

See issue #147.

Major

• Introduced the PackageWrapper and ResultWrapper subclasses.
  PackageWrapper will take a single plams.Job type as argument and automatically return the appropiate Package instance upon calling PackageWrapper.__call__().

>>> from scm.plams import ADFJob, AMSJob, Molecule
>>> from qmflows import Settings, PackageWrapper, run

>>> from qmflows.packages.SCM import ADF_Result
>>> from qmflows.packages.package_wrapper import ResultWrapper

>>> mol = Molecule(...)
>>> s1 = Settings(...)
>>> s2 = Settings(...)

>>> job_ams = PackageWrapper(AMSJob)(s1, mol, ...)
>>> job_adf = PackageWrapper(ADFJob)(s2, mol, ...)

>>> result_ams: ResultWrapper = run(job_ams, ...)
>>> result_adf: ADF_Result = run(job_adf, ...)

• Allow Package.generic2specific() to handle PLAMS style input settings.
  For example, settings.input.blablabla is now automatically converted into settings.specific[self.pkg_name].blablabla.

Minor

• Changed the Package.generic_dict_file attribute from an instance- to a class-variable, as it's value is already constant with respect to the class type.
• Changed the default value of Package.generic_dict_file to NotImplemented. Only applies to the Package baseclass, not its subclasses.
• Got rid of a lot of nesting in Package.generic2specific().
• Updated the Sphinx conf.py settings.
• Changed minimum Sphinx version to 2.1.
• Added documentation for the new PackageWrapper class.
• Added PackageWrapper to the main __init__.py file.
• Added new tests.
• Bug fix in init_restart() and InitRestart: only attempt to delete the newly created workdir if there is an actual previously existing workdir.

[felipeZ]

For the record, Here is a conversion with @BvB93 about the reason to create these classes:
"There are more Job classes in PLAMS than the corresponding Package subclasses in QMFlows. Those could, of course, be implemented, but that can be a bit of work (which may or may not be worth it depending on the situation) Enter PackageWrapper, which is a Package subclass that can work with any arbitrary passed Job type, albeit in a more barebones fashion
Even though, when the passed Job type is recognized it will return the appropriate Package instance anyway upon calling the __call__ method For example ADFJob will default to adf, Cp2kJob return cp2k, etc.."

[felipeZ]

Thanks for improving this mess!

[BvB93]

Continuation of #149 (comment):

Thanks for the info, I'll get back to you a bit later.

Allright, I've created a way to (sort of) mock the ADF results.
More specifically, it creates a normal-ish qmflows.Result instance and then attaches a pickled .dill file from a previous calculation.