Add some Laplace-Beltrami-type operators (#115)

* add some Laplace-Beltrami-type operators

* add test for Beltrami operators

* fix mean_curvature usage

* update test parameters to ensure some convergence

* add dim argument to beltrami operators

* add docs to beltrami operators

* add generic right-preconditioning

* add tests for 2d beltrami solutions

* beltrami: unify left and right preconditioning

* uses a sphere of radius 1.5