`ThPredictionsResult` should not use C++

Created by: siranipour

At the moment when computing theory predictions we make calls to C++

https://github.com/NNPDF/nnpdf/blob/70ca11df5c42079ef4d3daec774a4222473372a7/validphys2/src/validphys/results.py#L131

afaict this can be replaced in favour of validphys.convolution. In particular the stats no longer need to be done using C++ too. However, before fixing this I just want to make sure of the following:

When we have

https://github.com/NNPDF/nnpdf/blob/70ca11df5c42079ef4d3daec774a4222473372a7/validphys2/src/validphys/results.py#L108

Is this equivalent to:

np.std(self._rawdata, axis=1)

because at the moment, (for NMC for example)

np.allclose(self.std_error, np.std(self._rawdata, axis=1)
# False

cc @scarrazza