Implement HCP model for VocePL model

[rcarson3]

I was talking with @darrencpagan and he seemed interested in having this functionality later on down the road. So, I'll be using this issue as a mean to keep track of the progress of this feature request. Although, it might be a bit before I have the time to implement the feature.

So, I don't think this should be hard to implement especially since we really shouldn't have to implement any new features within ExaCMech. We just need the new slip kinetics class for the work, which shouldn't be bad.