Nexus: Adding crowds_serialize_walkers tag to dmc input list

[kayahans]

Proposed changes

Adds the crowds_serialize_walkers tag to the dmc input list in nexus.

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?

macOS Monterey 12.6.3, Python 3.9.13

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'
• No. 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)

[ye-luo]

Test this please

[kayahans]

I have missed adding the write type in the first commit. It should now write yes/no rather than True/False.

[prckent]

Since this is motivated by #4648, I'll note that crowds_serialize_walkers will little influence CPU performance but will, in general, destroy GPU performance. So this debugging flag should be used with care until we have a fix for #4648.

[mdewing]

Test this please