Reduce list of dependencies to a minimum, or add optional dependencies

[mbercx]

As @davidwaroquiers notes here in this comment, we currently have several dependencies that could potentially be removed:

• scipy 📦 Remove scipy dependency #96
• numpy: a bit more work, and quite ubiquitous in our ecosystem's packages in any case. Will look into it, but not a priority at the moment.
• xmlschema: @gbrunin I found your comment when cleaning up in b962e96. ^^ I'll look into it, but I think it's a Python-only package that only has one other dependency (https://pypi.org/project/elementpath/ - no dependencies). There is also https://pypi.org/project/qeschema/, which might be looking into to simplify our life even further. There is also https://pypi.org/project/lxml/, but not sure if that does what we need. Alternatively we could also skip schema validation, but I'm not sure that's a good idea. Sticking with xmlschema, a tool that is developed by the QE people, seems like the most reasonable approach at first glance.
• packaging: this was apparently introduced in e3f9744 as a way to deal with QE versions. Although I'm also wondering what do to with QE versions (which ones to support, how to deal with differences), this seems a little over-engineered to me. QE versions are not so complicated that they require an extra dependency to deal with them. Besides, even though QE versions look like SemVer, I don't they really adhere to it, and so trying to use PEP 440 to determine the right version might do more harm than good.

[mbercx]

Partially fixed by #96