Added support for "built-in" random distributions

This PR adds support for lightweight built-in random distributions that simply point to a definition at http://www.uncertml.org/distributions and define matching parameters in NineML syntax. I have also added all the different distribution types found at uncertml but we may want to truncate this list slightly if they do not appear in common maths libraries (although I think all bar one or two of them were in GNU GSL).