[sbml-required] Updates

[Lucian S. <lp...@sp...>]

I'm working on updating 'req' to use the new 'mathChanged' child, and I 
have a few other changes to propose, all visible at

https://docs.google.com/drawings/d/1tmE9ZorGlMlqnAkoln65ilIHcAkDLCKv5pfYl8TwUJ4

* That we allow more than one 'MathChanged' child, for situations where 
the math of an element is affected by more than one package (i.e. 
Spatial and Comp; or Distrib and Arrays)

* A change in the attribute names:

'mathOverridden' -> 'changedBy' (the fact that the math has changed is 
now encapsulated in the object name)

'coreHasAlternativeMath' -> 'viableWithoutChange'  (when multiple 
packages affect the same element, it's not core that needs to be 
checked; it's 'whatever is there minus this package' that needs to be 
checked.)

Any opinions?

-Lucian