Compute energy through a cross section

[ole]

As suggested by Rudy van Drie, there would be a case to calculate the average energy through a line. This could be used for Culverts instead of the current enquiry polygon and would have the advantage that the mesh doesn't need to be refined.

The formula for specific energy is E = h + v^{2/(2g) where h and v are average depth and velocity along the line segment.}

and the similar formula for total energy is E = w + v^{2/(2g) where w is average stage (w = z + h)}

This should be implemented reusing code developed for ticket:175

[ole]

Priority raised as this will also be important for understanding energy dissipation in the Hinwood wave breaking validation work. Does ANUGA dissipate energy and is there correlation with energy dissipation and breaking wave fronts?