Selections in configs

[rbarrue]

We want to be able to define selections in the configs and have naming of folders associated to these configs. These should be passed to all the jobs in the same config such that things are coherent. The RDataLoader and SelectionView objects accept selections via functions (SelectionFn objects).

To avoid unnecessary complexity, and also since we won't need to change the cuts too often, the idea is the following:

• define a set of named cuts, similar to what we have for observables
• have a small dictionary {cut_name: function, ...} to link strings and functions

Then one just needs to make sure that this is properly accessed.