testing for not None of thermo objects return by thermo estimator

added two assertions that assert that we always have a thermo object
to work with.
put dict outside function to so it doesn't rebuild every time.

also, provides a clearer overview of the supported error-cancelling
reactions.
removed loading other rmg databases besides thermo

we just load the thermo databases, the rest we don't need.
proof of concept of uncertainty checking for improved value of Df_H.

We compare the absolute deviation between the original value for the
unknown species and its 'improved' value.

If this deviation is smaller than the total uncertainty associated with
the 'improved' value, than we continue using it. Otherwise, we report it
loudly.