adding interlaced scanning schemes

[decarlof]

compute_positions_PSO()is currently not returning the angle vector which is fine for equally spaced angular data collection but I think one of the advantage of taking full control of the PSO is that we can generate other angular displacements like interlaced and, after rounding them to integer numbers of encoder counts, use them directly in new data collection schemes.

Any suggestion on where to implement these? If of general interest I think should go in tomoscan_pso.py