Import physical constants from qe-tools instead of aiida-core

[sphuber]

Fixes #69

The physical constants, mostly used by the parsers, used to be defined
in aiida-core for the Quantum ESPRESSO plugins, before they were moved
to this repository. The physical constants remained, however, but will
be removed for aiida-core==1.0.0. The constants had already be copied
to qe-tools as well, which is where they will stay. Here we change the
import to use the constants from the latter instead of aiida-core.

[sphuber]

I will open a PR on aiida-core as well to remove the constants from there. I was just thinking about compatibility though. Removing them from aiida-core will break existing aiida-quantumespresso releases. Given that they are alphas, that might be acceptable, especially if we make another release of aiida-quantumespresso, except we just released one yesterday. What do you think @giovannipizzi

[sphuber]

I also opened an issue on qe-tools to continue the discussion on the original problem of inconsistent use of constant sets.

[giovannipizzi]

ok with me. Anyway as I mentioned elsewhere, we should namespace the constants in qetools and get them from the correct namespace. Since the constants are already there anyway, ok for now as it is.
I think that when we change qe-tools, we will release a new version of it, and a new one of aiida-qe depending on it.

For the dependency of aiida-qe on aiida-core: the latest released version of aiida-qe should work with the latest released version of aiida-core. Also the develop versions should work together. Probably this is enough, but if it isn't, then we'll need to make a new release for both as soon as the change is applied? (anyway, maybe it's not critical, since in the new develop of aiida-qe we are not taking them from aiida-core anymore?)

[sphuber]

The only problem is that I should release a new alpha of aiida-quantumespresso that depends on qe-tools for the constants before a release is made of aiida-core where they are removed. Otherwise, if people upgrade their aiida-core, their current aiida-quantumespresso will fail. That is why I blocked the PR on aiida-core until after I made the latest beta release. That way we will have some period of leeway where we can apply the changes in aiida-quantumespresso and make yet another release there as well. It is unfortunate we just released an alpha on monday evening