Replace the GPfit package

[topepo]

It is fairly slow, requires indicators for categorical predictors, and the maintainers don't respond to emails.

I've been looking into GauPro.

Pros:

• it has kernel functions for categorical tuning parameters
• you can combine kernels (so that we can do different things for number and categorical kernels)
• fast
• allows updating a previous fit instead of refitting (but maybe on;y when there are numeric parameters)
• I've prototyped code to replace what we have in tune and it should be pretty easy.

Cons:

• Maybe still a work in progress? There are warnings when using it and a fair amount of browser() calls when there probably should be stop()s.
• Not documented to the degree that I'd like. I'd have to put in some issues to ask if we are doing it correctly.

I've put in an issue and PR for a simple bug that I found so we can gauge if the maintainer is active and collaborative.