Adds a new method: toMatrix()

In bug 3165 I write that I've notice that this method is missing even though it is used in a gist that is used by one of the QSAR cheat sheet. This pull request will add a method called "toMatrix()", that returns a string that, when printed, looks like a matrix.
I didn't to make a method that creates an IMatrixResource and returns that, 'cos it will arise an unwanted dependency to biocilpse.statistics.