Parallelism

Adds parallelism feature to g4camp with python's multiprocessing module

To run particles in multi processes, use --multiprocessing [-mpr] option, processes number can be changed with --process_num ... [-Npr ...] (4 by default)

Each process saves data about propagated particles until all processes have done, then the data is stored in .h5 file. In case of big amount of particles or/and their energy, this can potentially lead to crash, thus number of events per each process can be limited with --bunches_size ... [-Sbun ...] (1000 by default). If number of particles exceeds this, processes are started several times. Bunch -> processes -> events (N_events < bunches_size)