Prevent excess deepcopy using sbml io

This reduces read time by ~35% for Salmonella.
warn on more invalid formulas

The docstring has been updated to state that the only allowed characters
are letters and numbers. Warnings are explicitly raised for parenthesis
and '*' in a formula string.
SBML with metabolites on both sides of a reaction

This is case is now handled. The metabolite will not be added if it is
on both sides with the same stoichiometry.
Merge pull request #130 from aebrahim/loading_warnings

Small changes to SBML loading