Treatment of 3D results

[labrosse]

3D files can already be read. What is missing is:

• treatment of 3D spherical cases to go from the Yin-Yang grid to a r-theta-phi space.
• Iso-surfaces in cartesian and spherical geometry.
• Slices in cartesian and spherical geometry

[amorison]

Small step forward, the stagpy field command is now able to plot slices in 3D cartesian geometry. The position of the slice (an index) can be passed on with the --ix, --iy or --iz command line options. Implemented in c23a2d0.