as.im3d.matrix should round away floating point errors

[jefferis]

See #263

also revealed by

vim=as.im3d(xyzmatrix(v.jfrc2), voxdims(JFRC2), BoundingBox=boundingbox(JFRC2))

although the

[jefferis]

This is actually more complicated than I thought – think about the case where points are far outside the specified volume – presumably we don't want to keep then. But what about points near the volume? Perhaps we do want to keep them if they are e.g. within 1/1000 or 1/100 of a voxel.