Unit tests

[alecksphillips]

Add unit tests for existing components and all subcomponents (e.g. eval, sample, etc):

Base:

• Random
• Executor
• Executor_MPI
• Stan Model (compilation, logprob and gradient eval)

Types:

• Decision Trees
• Regression Trees
• Additive Structure
• Spectrum

Algorithms:

• MCMC
• SMC
• RJMCMC

SMC components:

• L-kernel
• Effective sample size
• Resampling
• Redistribution
• Weight normalisation