In the code for particle matter interaction boundp was called when more

than zero particles are outside the material. When a single particle was
outside the material OPAL aborted with the message 'h<0, can not build a
mesh'. The bounding box around a single particle has no volume and hence
the volume of the mesh would be zero as well. This is fixed if more than
one particle is outside the material. For consistency with the method
computeSpaceChargeFields I chose minBinEmitted_m as number of particles
that have to be outside. Its default value is 10.