Nexus: Better handling tiling matrix

[kayahans]

When general 3x3 tiling matrices are used, existing conditionals do not work as intended.

Here is the error message I received when using a 3x3 numpy array as a tiling matrix:

 File "/ccsopen/home/ksu/SOFTWARE/qmcpack/latest/qmcpack/nexus/lib/physical_system.py", line 739, in generate_physical_system
    if tiling!=None and tiling!=(1,1,1) and structure.has_folded():
       ^^^^^^^^^^^^
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

When comparing to None, it is recommended to use is or is not.

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

• Bugfix

Does this introduce a breaking change?

• No. All tests passed with nxs-test

What systems has this change been tested on?

MacOS 15.3.1

Checklist

• 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)

[prckent]

LGTM

[ye-luo]

Test this please