[ck2cti] Partial support for scientific notation in stoichiometric coefficients

This won't work for positive exponents, because the '+' will still be
interpreted as a delimiter between terms of the reaction expression.

git-svn-id: https://cantera.googlecode.com/svn/cantera/branches/2.1@2562 02a645c2-efd0-11dd-984d-ab748d24aa7e