49th example -outputing potential and field data for later re-use

Same geometry as xmpl2d07, but without automatic iteration to optimise a focus, and with outputting of data for re-use as imported data for xmpl2d50.

The data for re-use is in a grid that covers the region x,y = 0 to 0.13, z = -4.05 to 0.

The grid has potentials and fields and is output to the ray output file temp49a.dat.

These potentials and fields are re-used in xmpl2d50.