Prepare for HSpice Simulator

[AidenDawn]

I have been working on getting HSpice working in PySpice. This is the minor downstream changes needed to start getting HSpice working. See PR #5.

[infinitymdm]

YAML configs in the test directory are present for one of two purposes:

1. For testing CharLib in CI
2. For examples in documentation (which will also eventually be tested in a forthcoming CI workflow)

Unfortunately, there's no practical way to include hspice testing in our CI. For this reason, I'm requesting two changes to this PR:

• Remove the two hspice-specific YAML files under test/examples
• Modify the description for the backend config key to make it clear that hspice is an experimental backend with no guarantee of functionality. (Feel free to remove the link to the PySpice docs too, it's outdated and not particularly helpful.)

[infinitymdm]

Will be merged following infinitymdm/PySpice#5.