Re-factored the whole external potential mechanism to match what is done for cooling

Also, renamed 'disk' to 'disc' and improved the documentation and description of the parameters.

@pdraper this is just code rearrangement, no changes to the actual physics has been done.

doc/Doxyfile.in

@@ -763,6 +763,7 @@ INPUT                  =  @top_srcdir@ @top_srcdir@/src @top_srcdir@/tests @top_
 INPUT		       += @top_srcdir@/src/hydro/Minimal
 INPUT		       += @top_srcdir@/src/gravity/Default
 INPUT		       += @top_srcdir@/src/riemann
+INPUT		       += @top_srcdir@/src/potential/point_mass
 INPUT		       += @top_srcdir@/src/cooling/const_du
 
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