Loader should have a check_experiment method

Created by: siranipour

Currently, if I want an NNPDF::Experiment object, I have to to have a horrible roundabout way of doing it:

In [1]: from validphys.core import ExperimentSpec                                  

In [2]: from validphys.loader import Loader 
   ...: l = Loader()                                                                                                                      

In [3]: spec = ExperimentSpec("Spec", [l.check_dataset("NMC", theoryid=53)])       

In [4]: spec.load()                                                                

-- Reading COMMONDATA for Dataset: NMC
nData: 292 nSys: 16
-- COMMONDATA Files for NMC successfully read.

_NMC________________________________________________________
NMC
292 Data Points 50 X points 9 active flavours
Out[4]: <NNPDF.nnpdf.Experiment; proxy of <Swig Object of type 'NNPDF::Experiment *' at 0x7f3be614a690> 

Wouldn't it make more sense to have a loader.check_experiment method to return the c++ object we're after here?