Changed the underlying map to be a Map<scalar> to allow the addition
of a value to the Distribution to be weighted (by timestep or by cell
size for example) component by-component - default weight is one.
Adding normalisation, write, mean snd median functions.
Test application DistributionTest draws numbers from the Random class
and creates distributions of them, writing to disk, for the main
types: scalar, vector, symmTensior, sphericalTensor and tensor.
Creating a labelVector distibution, but need to check what is
happening to the mean and median values and what happens when an odd
number is used as a binWidth, as truncation to label will be
occurring.
