Changed lambda_j to xi in test code. Updated test results. Added assertthat and tensorA to imports. Reduced no. iterations mcmc example.

Updated R version as tensorSlice funtion did not work correctly in earlier versions. Changes to tensorSlice made in previous commit undone as R update corrects the problem.

Tidied up, and documented DAG and MCMC framework.  model.R contains the SA model, built using the DAG.  test.R contains test functions.

Converted to R6 classes.  Still having trouble sampling from the Dirichlet distribution with the R's.

Tidied library dependencies, making use of the "Imports" statement in DESCRIPTION, rather than calls to library(...) all over the place.