Minimum length of field lines

[dschwoerer]

If no special care is taken, grids can contain field lines, that are only on a single y-slice.

Refining the grid in y does in general not help, the best solution is probably to exclude such cases in zoidberg.

Otherwise the boundary condition code can only rely on a single point, thus strongly limiting what extrapolations can be done.

[bshanahan]

A minimum field line length is probably the best solution, but one could also check that a few extra guard cells wouldn't catch the field line.