Nexus: Add Lowdin example to nexus/examples

[mcbennet]

Proposed changes

The following PR adds two examples to nexus that calculate Lowdin charges. For diamond.py nspin=1 is specified in the QE run such that there is only 1 SPO. For diamond_spin.py nspin=2 is specified in the QE run such that there are 2 SPOs. In both cases, lowdin.py can be used after the 1RDM of QMCPACK is generated to calculate the Lowdin charges for the AO basis attached to the .UPF file.

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

• New feature

Does this introduce a breaking change?

• No

What systems has this change been tested on?

QMCPACK Workshop VM

Checklist

Update the following with a yes where the items apply. If you're unsure about any of them, don't hesitate to ask. This is
simply a reminder of what we are going to look for before merging your code.

• Yes. This PR is up to date with current the current state of 'develop'
• N/A. Code added or changed in the PR has been clang-formatted
• No. This PR adds tests to cover any new code, or to catch a bug that is being fixed
• No. Documentation has been added (if appropriate)

[qmc-robot]

Can one of the admins verify this patch?

[ye-luo]

Test this please