Add jinja2 block around memory options

[cbkerr]

Motivation and Context

This is to enable overriding --mem with --mem-per-gpu or other cluster-specific options.
See discussion at #819 (comment)

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• The changes introduced by this pull request are covered by existing or newly introduced tests.
• The package documentation and framework documentation in signac-docs are up to date with these changes.
• I have updated the changelog and added any related issue and pull request numbers for future reference.

[joaander]

Thanks!

@bcrawford39GT this should allow you override the --mem in the next release of signac without waiting for #819.