Exposition of the parallel-distance-matrix.cpp Gallery Entry

Here is an edited parallel-distance-matrix.cpp with background text, comments and benchmarking code to demonstrate the use of RcppParallel on row-based computations. 

At the moment, this code demonstrates how one could use almost any distance metric by editing/adding additional parallel workers. One refinement of this code would be to take a distance kernel in the parallel worker constructor which is the kernel.