[newchem-cpp] address compiler warnings

[mabruzzo]

This PR addresses 3 minor compiler warnings:

1. it didn't like that we assign scoef to itself in chemistry_solver_funcs.hpp
2. there was a second occurrence of that first warning
3. it didn't like that ttmin was uninitialized
   • the compiler didn't like this because we could theoretically reach a print statement that tried to print ttmin before we actually defined ttmin
   • I'm pretty sure that in practice this situation was unreachable, but I decided to satisfy the compiler

I also adjusted the boilerplate at the top of chemistry_solver_funcs.hpp