Go HEP release 0.4

Release v0.4 is a small iteration (because of holidays.)

But still, it does bring interesting things to the table:

• root2npy, a new command to convert the content of simple TTrees into a NumPy data file,
• migration from the github.com/gonum/xyz packages to their new location – gonum.org/v1/gonum/xyz,

fastjet

• preliminary work for implementing better/faster jet clustering, thanks Bastian Wieck

hplot

• make the hplot.H1D type implement gonum/plot.Thumbnailer (for better integration with legends and labels)

lcio

• fix a bug in the marshaling of lcio.TrackerRawData, thanks David Blyth