Updated math code in libSBML

[Sarah Keating]

Hi LibSBML-users

**FEEDBACK REQUEST**

Previously libSBML had included an implementation of the ASTNodes to
facilitate extension by packages. This had an umbrella class to preserve
backwards compatibility - and unfortunately created an unacceptable
impact on computation speed for users who make use of the ASTNode class
beyond just reading and writing. Hence we have since included 'legacy
math' as option within the libSBML release.

However, as more packages start to extend the math available, we need to
resolve the issue and have been working on producing an implementation
of ASTNode that will accept and use plugins with minimal impact on
performance. This code is currently in our svn repository. It would be
really useful if people who were affected by the previous performance
hit could test this code out and see let us know whether the current
code is acceptable.

(There is a slight performance difference see
https://docs.google.com/spreadsheets/d/13rc2jO61gkR8Ifj8-tZ9a0wzIeeXtc1SJYn5FKgDx54/edit?usp=sharing
for our tests. Note old-legacy means the math-legacy code as in last
release; new-math is math code as in last release and legacy-plugin is
current svn code).

If you need a specific binary build to test with; please let us know and
we can make that available.

Thanks

Sarah