Adding biharmonic energy function

[odedstein]

Adding a function to implement the biharmonic energy, both intrinsic and extrinsic.

Also adding grad_intrinsic, following the gptoolbox example.

[sgsellan]

Thanks! Some questions before merging:

1. Could you improve the first docstring line in "biharmonic energy"? Specially since this is not actually computing an energy, rather, building a matrix. Maybe something like "Returns a matrix Q such that if u is a vector of vertex samples of a function f on the mesh V,F, then u'Qu is an approximation of the integral \int (laplacian (f))^2 (if energy='...') or \int ||Hessian(f)|| (if ...) ...

2. We usually have the convention of starting with underscore functions that are only used internally in some other function. Does it make sense to change hessian_energy to _hessian_energy?

3. Could you double-check the docstring in biharmonic_energy_intrinsic? There may be some copy-paste issues I think.

4. I pushed minor changes making it consinstent that we add in the "Notes" where we got the function from.

5. What is up with test_single_triangle_2d in the test_biharmonic_energy script?

[sgsellan]

^ if you feel like you've addressed those, feel free to merge. Everything else looks fine

[odedstein]

Thanks! Some questions before merging:

1. Could you improve the first docstring line in "biharmonic energy"? Specially since this is not actually computing an energy, rather, building a matrix. Maybe something like "Returns a matrix Q such that if u is a vector of vertex samples of a function f on the mesh V,F, then u'_Q_u is an approximation of the integral \int (laplacian (f))^2 (if energy='...') or \int ||Hessian(f)|| (if ...) ...
2. We usually have the convention of starting with underscore functions that are only used internally in some other function. Does it make sense to change hessian_energy to _hessian_energy?
3. Could you double-check the docstring in biharmonic_energy_intrinsic? There may be some copy-paste issues I think.
4. I pushed minor changes making it consinstent that we add in the "Notes" where we got the function from.
5. What is up with test_single_triangle_2d in the test_biharmonic_energy script?

I addressed all of these. Will merge if the checks pass.