Specify `numpy.ndarray` shapes

[redeboer]

Specify numpy.ndarray shapes with nptyping.NDArray

[spflueger]

With the move to sympy, the lambification process to an evaluable function makes the type hinting impossible at some point.

[redeboer]

For the computation of the helicity angles (which can't be done in sympy for now), I would still like to see some type hints for the shapes. But afaics, this is not possible with numpy and nptyping is buggy. Alternative would be to wrap, say, an np.array of shape (n, 4) (for a four-momentum tuple and n events) into some LorentzVectorData class.