HAL component for tangential knife

Thanks, I don't know how good a teacher I will be...
I'm almost there on my state_tag_arcs branch at github.com/rodw-au/linuxcnc

I've got all of the calculations tested and working in the interpreter. For some reason, data is getting lost on its way to motion. I will troubleshoot that in the next day or so.

Its been more complicated than I remember. I've got a better method of setting the tags but they are only updated at the beginning of a gcode segment but we want to get real time headings all the way around a G2/G3 arc. To do this, I needed to calculate the heading in real time (eg with the servo thread) in motion. No wonder no-one has tried to do this before!