Handle disabled variational parameters in rotation derivatives

[mdewing]

Disabled variational parameters will return -1 from a call to myVars.where, leading to an out-of-bounds array access. This PR adds a check to avoid this situation.

Variational parameters can be disabled due to choosing a variational subset.

Also in the case of a RHF wavefunction where there are different numbers of spin up/down electrons, there will be a different number of rotation parameters for each spin. At a higher level of managing the parameters, this case is not being handled correctly, but this fix will stop it from segfaulting on invalid array access in the meantime.

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

Delete the items that do not apply

• Bugfix

Does this introduce a breaking change?

• No

What systems has this change been tested on?

desktop

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. This PR adds tests to cover any new code, or to catch a bug that is being fixed
• N/A. Documentation has been added (if appropriate)

[mdewing]

The RHF case is described in #4582

[mdewing]

Test this please