PartonicChannel caching

[alecandido]

The cache at ESF level is not very effective

• we might evaluate its impact just deactivating it and running, but I guess it's no more than a factor of two

but we could move the caching at the level of PartonicChannel, once we reworked the Info #126.

PartonicChannels are different only based on their path inside the package (that identifies the expression) and Info passed (at the moment a lot, but we'll hugely restrict).
So, for example, even completely different Kernels might share the pc, just having different weights.

• We could simply compare the path and the Info hash. We combine into a single hash and we're done :)