Save walker weights in HDF5 files

[quantumsteve]

Please review the developer documentation
on the wiki of this project that contains help and requirements.

Proposed changes

Describe what this PR changes and why. If it closes an issue, link to it here
with a supported keyword.

Saves the walker weights following the same pattern used to save the particle positions.

This fixes #4157

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

Delete the items that do not apply

• New feature
• Refactoring (no functional changes, no api changes)

Does this introduce a breaking change?

• No

What systems has this change been tested on?

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

[ye-luo]

Could you document the h5 data layout in the manual.

Mention the layout of offsets, electron positions and weights dataset.

[quantumsteve]

Could you document the h5 data layout in the manual.

Is this the best place to document the .config.h5 file? Is it documented elsewhere?
https://github.com/QMCPACK/qmcpack/blob/507d0de7dcdbee1b1714b57fb7b688e96586fb36/docs/output_overview.rst#the-configh5-file

[ye-luo]

Could you document the h5 data layout in the manual.

Is this the best place to document the .config.h5 file? Is it documented elsewhere? https://github.com/QMCPACK/qmcpack/blob/507d0de7dcdbee1b1714b57fb7b688e96586fb36/docs/output_overview.rst#the-configh5-file

I don't have a better place. Just do it there.

[quantumsteve]

Could you document the h5 data layout in the manual.

Is this the best place to document the .config.h5 file? Is it documented elsewhere? https://github.com/QMCPACK/qmcpack/blob/507d0de7dcdbee1b1714b57fb7b688e96586fb36/docs/output_overview.rst#the-configh5-file

I don't have a better place. Just do it there.

I dumped the contents from one of our restart tests. Still need to describe the individual groups and datasets.

[ye-luo]

Test this please