Fix factorial2

[FarnazH]

Steps

• Write a good description of what the PR does.
• Add tests for each unit of code added (e.g. function, class)
• Update documentation
• Squash commits that can be grouped together
• Rebase onto master

Description

This PR addresses issue #129 by adding a factorial2 function that wraps scipy.special.factorial2 to return 1.0 when the input is not positive. This is temporary additional, while we wait for Scipy's update. To learn more, see scipy/scipy#18409. This function is added in gbasis.utils (I tried adding it to gbasis.wrappers first but that would cause circular imports).

Just patching scipy version (as was done in f74123e) was not enough, because unless you install the gbasis package, the correct version of scipy is not used causing headache. Also, one might need to use scipy 1.11.x in their environment for other purposes. In the meanwhile, correcting the factorial2 output seems to me a more practical solution than patching the scipy version.

Type of Changes

	Type
✓	🐛 Bug fix

Related Issue

Closes #129

[PaulWAyers]

Looks good to me. Thanks @FarnazH

Are there any other pieces of gbasis that really belong in utils.py? I can't think of any right now, but probably eventually some other things (e.g., functions related to the Boys function) belong there.

[FarnazH]

@PaulWAyers, I cannot think of any other function right now, but it is good to keep in mind.