Updating tunneling to allow calculations for symmetric Eckart

Allowing for symmetric eckart tunneling. In symmetric eckart
we assume the reactants and product have the same energy, therefore
the terms
dV1 = E0_TS - E0_reac
dV2 = E0_TS - E0_prod

will be equal. To allow for this I have relaxed the assertion
dV1 < dV2 to dV1 <= dV2.

It works fine for my test reactions. I would assume the assertion
was originally placed anticipating so as not to run into
a numerical issue (division by 0).
Merge pull request #222 from shamelmerchant/master

Updating tunneling to allow calculations for symmetric Eckart