Handle `hops` parameters

[e00E]

There are still two url parameters from the nodejs price estimator that we haven't implemented: atoms=true and hops=

hops doesn't apply to the pricegraph so I think we should remove it.

atoms always has to be set to true for the nodejs price estimator so we can either enforce this here too or ignore the parameter (currently it is being ignored). I'm not sure what the expected behavior is supposed to be with atoms=false for which use case we want this.

From what I remember, the atoms parameter was supposed to specify the unit of the estimated buy amount. Since we currently only support atoms as the units, I think if atoms is false or missing, we should return an 501 Not Implemented HTTP error.