Enable use of mlpack machine learning library in stochastic tools module

[aeslaughter]

Reason

Users have interest in using machine learning for stochastic analysis directly within MOOSE, in C++.

Design

Optionally link mlpack (https://www.mlpack.org) to the stochastic tools module to allow users to create custom objects using all available features in the library

Impact

This will enhance the public API of stochastic tools to include mlpack

[aeslaughter]

I was able to get this working on a local machine, but future efforts are likely to use pytorch.