Bug correction: injected particles were initialized with the wrong charge sign. Attention to the use of the fabs(qom)/qom.

Adding getBx(i,j,k). Adding an option to read the Bext from a file. Adding the initialization of drift velocities using the initial ExB. Particles are not saved at the initial iteration anymore (it uses too much disk space in some systems).

This version has been tested with external hdf5 fields initialization.