[WIP] Reader proposal for the commondata proposal

Created by: scarlehoff

In the root of the repository there is a jupyter notebook that should be able to work with this branch of the code.

I made two small corrections to the proposal yaml/json files (I commented about them in the other PR). The only actual change is the variants of course.

What I did:

Added a KinematicsSpec and UncertaintiesSpec. These two are loaded inside the commondata file when the user asks explicitly for kinematics or uncertainties since they might be changed by the metadata.

At the moment CommonDataSpec takes as input the datafile (as before) but in the light of the discussion today maybe the loader should only read up the metadata (and the information of dataset_input) and everything else done afterwards.

dataset_input gains a new key: variant

The CommonData coming from libNNPDF has ben removed. The new format I think might please @Zaharid since now not only nothing gets loaded until the user explicitly ask for it but if the user only needs, say, the data (no uncertainties), the kinematics or the uncertainties, that's the only thing that will get loaded. Instead, in the current version, CommonData had to load everything.

Note: the "new loader" already works by setting the environment path BUILDMASTER_PATH to wherever the commondata folders are, since at the moment they are not installed.