Add He3 tube kernel

[yxqd]

C++ implementation: https://github.com/mcvine/mcvine/blob/master/packages/mccomponents/lib/kernels/detector/He3Tube.h
https://github.com/mcvine/mcvine/blob/master/packages/mccomponents/lib/kernels/detector/He3Tube.cc

• Each tube has a tuple of "channel numbers". For example (19, 2).
• The absorb function calculates the pixel index along this tube, for example 101, and tof index, 2345, then store the tuple of tube, pixel, tof channel numbers, e.g., (19, 2, 101, 2345) to a "MCA".

python layer to convert python representation to c++ objects:
https://github.com/mcvine/mcvine/blob/master/packages/mccomponents/python/mccomponents/detector/ComputationEngineRendererExtension.py

The "channel numbers" for a tube is given by _indexes_in_detsys in
https://github.com/mcvine/mcvine/blob/master/packages/mccomponents/python/mccomponents/detector/ComputationEngineRendererExtension.py#L123

In order to support detector system simulation, we need to support composite