Add viscous and resistive terms to fluid and MHD models

Solves the following issue(s):

Closes #279 (closed)

Core changes:

Add a possible offset to p0 in screw pintch equilibrium.

Fix an indentation error for the evaluation of fields in psydac_derham.py/eval_tp_fixed_loc.

Model-specific changes:

Added a new propagators taking care of viscous and resistive effects.

Two new models : ViscousFluid and ViscoresistiveMHD.

New equilibrium corresponding to a fluid with constant pressure and a layer with different density (usefull to run shear layer test simulations).

Split the entropy interface in equilibrium to have to possibility to have a gaz with \gamma = 7/5 (previously was only 5/3)

Documentation changes:

Added documentation for the new propagators and new models.