Add to_numpy/from_numpy functions

[APN-Pucky]

This is a preparation for hdf5 output (#369), since the data will be strictly rectilinear arrays (if I am not mistaken events have a fixed number of particles in HDF5, like evtgen format?) and h5py.File can write numpy arrays directly.

Parsing a hdf5 input file would probably go h5py <-> numpy <-> pylhe.

[eduardo-rodrigues]

Related to this idea, or superseding it, it might be worth having a discussion among Scikit-HEP orgs for opinions on what to provide. Similarly to what Vector has, a "transformation" to awkward arrays is likely the best thing to do. Conversions to NumPy and various other bits of functionality would then come along automatically/automagically (even histogramming).

Related to #300.

[APN-Pucky]

Yes, I decided against numpy -> hdf5 because h5py supports streaming/generator yields which is very similar to the xml parsing of normal lhe files.