Fix indexing error in ImplicitSurfChem::eval
Fix indexing error in ImplicitSurfChem::eval

This method was not working in the case where there was more than one surface
phase, since 'loc' was always set to zero.

imitrichev/cantera,imitrichev/cantera,imitrichev/cantera,imitrichev/cantera,imitrichev/cantera,imitrichev/cantera