change some of the smiles in the chb test

rmg-py generates other smiles strings, leading to KeyError's when comparing.
remove exclude_terminal_atoms() method

this was already covered by the n = no. of heavy neighbor - 1 method
add isTerminalAtom(atom) method
move account_for_branching() method to module level
do not include 'terminal bonds' in cbh-2 reactant population
factor out isTerminalBond(atom1, atom2) method

makes it more readable
make check() method more symmetric

before, rxn.coefficients could have had elements that d would not have,
and it would remain unchecked.
corrected error in reaction string
add CBH3Reaction class and populate_products method.