[losterbr]

PR title

Add tests for JointStochasticProcess

Summary

• Add a test implementation of the abstract JointStochasticProcess hooks and simple component processes.
• Cover combined process methods, covariance and diffusion, evolution, correlation caching and invalidation, state-dependent correlation, constructor/time preconditions, zero volatility, and rank-deficient factors.
• Register the new test source in CMake, Automake, and Visual Studio project files.

Notes for Maintainer

-This was previously neither used nor tested in existing code base. An alternative would be to remove this file.