nagyprism_potential

A function to perform terrain contribution to the potential using the Nagy prism formula.
To calculate the terrain contribution to the potential for a prism mass
with coordinates in the x,y,z plane and
%.........................................
%................._____________(x2,y2,z2).....
%............... /!................... ./!..............
%.............../_!___________/.!..............
%...............!.!....................!..!..............
%...............!.!....................!..!..............
%...............!.!....................!..!..............
%...............!.!....................!..!..............
%...............!.!(x1,y2,z1).....!..!(x2,y2,z1)....
%...............!./...................!./...............
%(x1,y1,z1)!/___________!/(x2,y1,z1)......

with density rho; the terrain potential T is given by
T = nagyprism_potential(x1,x2,y1,y2,z1,z2,rho)