Added option for geodesic curvature in the GaussCurvature functional

[joshichaitanya3]

This PR adds the option of setting a flag geodesic to be true in the GaussCurvature functional object. This is useful to compute the Gaussian curvature integrals for open surfaces using the discrete Gauss-Bonnet theorem. This allows us to do something like

var mesh = Mesh("disk.mesh")
mesh.addgrade(1)
var bnd = Selection(mesh, boundary=true)

// Geodesic curvature
var lgc = GaussCurvature()
lgc.geodesic = true
print lgc.total(mesh, selection=bnd) //expect 2*Pi * Euler characteristic, = 2*Pi

[joshichaitanya3]

Hmm, the geodesiccurvature test is passing on my laptop. Will have to look into it further.

[joshichaitanya3]

Turns out I forgot to add the disk mesh file :/
Passing the tests finally.

[joshichaitanya3]

I will add documentation and more tests later today.

[joshichaitanya3]

I have now added documentation and updated the test. The geodesic curvature calculation is now tested on a flat disk, an open tube and a hemispherical shell.