Inheritance Structure

I've put a great deal of thought into how to arrange the inheritance structures so as to get the most out of polymorphism. This is hindered somewhat by the fact that Fortran doesn't have a concept of interfaces like that of, say, Java. These are useful because they allow something along the lines of multiple inheritance and a greater degree of polymorphism. In any case, the inheritance structure I've come up with can be found in [the wiki](https://github.com/Fortran-FOSS-Programmers/FIAT/wiki/Type-Hierarchy). If anyone has any suggestions on how to improve it, that would be great.