Changes proposed in this pull request

Currently, a constant kinetics object cannot be used to obtain the Arrhenius parameters of falloff reactions in the C++ interface without creating a copy of the reaction. Let's assume I need to access the Arrhenius parameters of a falloff reaction. I am working on a constant reference of the kinetics object:

void f(const BulkKinetics& kin) // pass the kinetics object as const ref
{
    // I want to access the Arrhenius parameters of the first reaction,
    // which I know is of type falloff
    const ArrheniusRate& rate = std::dynamic_pointer_cast<const FalloffRate>
                                 (kin.reaction(0)->rate())->lowRate();
    // ERROR: lowRate() is not a constant member function
}

To address this, constant variants of the lowRate() and highRate() member functions are added in this PR.

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review