The various ways to crash!

PTC can be forced to stop execution for several reasons:

1. Physics: things get too big for the apertures-- physical aperture set by the users on each magnet or the global aperture absolute_aperture.
2. Things are not well defined mathematical: square roots of negatives, exponential which are too large, arcsin of numbers greater than one, etc...
3. The Taylor series package tries to do crazy things.
4. Normal forms, which are essential part of accelerator theory applied to tracking codes, can go wild. no_hyperbolic_in_normal_form will prevent the normalizaton of a plane into $x^2-p^2$. That is to say that one can reject hyperbolic fixed points.