usage showfield_general(z,loc);
displays fields/potentials specified z in channels at locations
specified in locs as a contour-plot
pars is optional
pars.scale sets the scale of the color map. Either a 1x2 vector
corresponding to minimum and a maximum or just a number (say x)
then the scale is from [-x x]. The default is
scale=[ -max(abs(z)) max(abs(z)) ]