Move calcfunctions to aiida_quantumespresso.calculations.functions

[sphuber]

Fixes #518

Since they are calculation functions they do not belong in the workflow
module. To not break backwards-compatibility the original modules import
the functions from their new location and emit a Warning. Note that we
explicitly do not use a DeprecationWarning since those are
automatically filtered in Python 3.

[greschd]

I think that should be a FutureWarning, based on how it's described here: https://docs.python.org/3/library/exceptions.html#FutureWarning.
Of course DeprecationWarning will still be shown e.g. by pytest, but I agree we are closer to an "end-user" scenario here, since we don't expect all code using it to run through a test suite.

I checked that FutureWarning is not silenced by default in py3.

[greschd]

The test failure is on me, the test should probably filter out calcfunctions 😅

[sphuber]

I think the way the words "user" and "developer" are being used by the Python docs is a bit different from our usage though. I want this warning to be seen by people using the aiida-quantumespresso plugin, but really they will be "developing" their own plugin or at least AiiDA-based code. I am guessing their reasoning is that developers will be reading the source code of libraries they are using (optimistic but okay) and so they will see it there and therefore it can be suppressed by default. My interpretation of the FutureWarning is that if you need to warn about deprecations that will even reach end-users. But for this change here, I need to reach everyone. But anyway, in the end it does not really matter. I will use FutureWarning given that it is more specific than just a general Warning. Will also fix the test :)

[sphuber]

@greschd fixed the tests and change the Warning to FutureWarning