Separate public / private interface.

[greschd]

Fixes #41, fixes #42

Separate the public / private interface and rename as discussed in #41

As discussed in #42, the duplicated code from CpInputFile and PwInputFile has been moved into the common base, which was renamed and made private.

The constants are now given as a SimpleNamespace, instead of defined at the top-level of their module. The reason for this is that we want to be compatible with adding version-specific constants later on. This could now be done by adding a get_constants function that takes the QE version as input, and returns the SimpleNamespace of corresponding constants.

25 is also related, but not completely fixed

Next steps:

• I will test this locally with https://github.com/aiidateam/aiida-quantumespresso/pull/503
• Merge this PR and release rc1
• Merge https://github.com/aiidateam/aiida-quantumespresso/pull/503

TODO for 2.0 release proper:

• Check the constants, see #25.
• check and update docstrings
• add (minimal) documentation

[giovannipizzi]

Of course I'm happy to re-review if you want to address the remaining points before

[greschd]

Of course I'm happy to re-review if you want to address the remaining points before

Nah, I've made separate issues for that.