Improve fortran assert function for debubugging

[AdityaSavara]

Currently, kmc_model.compile_options = '-d' turns on the assert statements, but they do not work while running a model in a typical installation. They may need some kind of special fortran environment (I do not know which one is needed).

I think the assert statement should be made into a 'regular' fortran function rather than in a ppc file so that it can work in the normally used environment.