PIERNIK mhd code - a multi-fluid, non-ideal extension of the
  relaxing-TVD scheme (IV)
  We present a new multi-fluid, grid MHD code PIERNIK, which is based on the
Relaxing TVD scheme (Jin & Xin, 1995). The original scheme (see Trac & Pen
(2003) and Pen et al. (2003)) has been extended by an addition of dynamically
independent, but interacting fluids: dust and a diffusive cosmic ray gas,
described within the fluid approximation, with an option to add other fluids in
an easy way. The code has been equipped with shearing-box boundary conditions,
and a selfgravity module, Ohmic resistivity module, as well as other facilities
which are useful in astrophysical fluid-dynamical simulations. The code is
parallelized by means of the MPI library. In this paper we present an extension
of PIERNIK, which is designed for simulations of diffusive propagation of the
Cosmic-Ray (CR) component in the magnetized ISM.
