Nexus: Add dependency versions file + update docs

[aannabe]

Proposed changes

This PR should close #5253. A requirements.txt file was added in qmcpack/nexus/. I did not add requirements_minimal.txt as only numpy is needed for core functionality and adding a file for just a single package seems redundant to me.

Appropriate description has been added to the Nexus documentation. Other smaller issues were also fixed in the docs: python3 support, and --user instead of -user.

What type(s) of changes does this code introduce?

• Documentation or build script changes

Does this introduce a breaking change?

• No

What systems has this change been tested on?

Local Linux Machine

Checklist

• Yes. This PR is up to date with current the current state of 'develop'
• Yes. Documentation has been added (if appropriate)

[prckent]

Please add requirements_minimal. The intent is to advertise to people that the full set of requirements is not needed. e.g. if spglib breaks, or so that someone using a different package manager can see that everything is not absolutely required. If requirements_minimal is not present people will think that all the packages are required, which is not the case.

[aannabe]

requirements_minimal.txt with the same header but only with numpy is now added.

[prckent]

Thanks Gani!

[prckent]

Test this please