augment continuity options

[Stephen Siegel]

An abstract function has a continuity field associated to it. This is the number of times the function can be differentiated. This must be improved in two ways:

1. Make continuity optional. Some abstract functions do not have any notion of continuity. (They don't have to be functions of real numbers, for examples). In this case, continuity could be "null" and the TASS abstract pragma should not have to include continuity.

2. Allow infinitely differentiable functions. So continuity can be a special value, INFINITY. Think of how to represent this in the TASS pragma. \infty?

Changes will be need to be made to model level, front end, and perhaps other modules.

[zirkel]

For infinitely differentiable functions, what about having a different keyword instead of a special value? Maybe something like

#pragma TASS abstract smooth double u(double x);