Field fixes

[jobh]

This fixes some "user errors" I have encountered. Like user setting dz=0 (error) or dz=1 (too small step size when dy/dx are large) in 2D. Or segmentation fault when stepping into zero-velocity region.

[malcolmw]

Thanks, @jobh for these fixes! And good catch on the node_interval <= 0 test. I am going to merge these.