Support SOFIE for MVA evaluation

The user interface is similar as for the other MVA, just use it as:

nn1 = mvaEvaluator("sofie_model.hxx")
nn1(mu.pt, mu.eta, mu.phi)

The generated .dat weight file should be in the same folder as the .hxx file (otherwise, specify its path as otherArgs in mvaEvaluator).

For converting an ONNX/PyTorch/Keras file to SOFIE, see the ROOT tutorials.

Closes #99 (closed)

Still TODO:

• test it!
• add tests tests will be added later, when the MatMul operator used in the nn1.onnx used in Bamboo's MVA tests is integrated into ROOT
• update documentation

cc. @kjaffel @fbury