Moved the signal velocity calculation to a new file that is SPH-scheme independant. There are two versions in there, one for MHD runs, one for non-MHD runs
Added the mhd directory for the scheme specific things. In there there is only None scheme for now, i.e. no-MHD runs
Added all the empty functions for the loops over neighbours
Added all the empty functions for the things done in-between loops (drift, kick, reset...)
Added a small structure that is carried by the particles to contain the MHD fields.
Todo:
Deal with the case where the MHD wants three loops but the hydro only needs two...