Implement simple Laplace solver and some related utilities

[amgebauer]

This PR adds a basic Laplace solver and some utility functions that are useful for computing spatially varying fibers in the context of rule-based methods. This includes:

• laplace.solve(...), laplace.solve_onezero(...): Solve a basic Laplace problem on the mesh
• fe.grad(...): Compute the gradient of a scalar field
• merge(...): Merge multiple mesh objects into a single one
• filter(...): Filter subsets of a mesh
• utils.bislerp(...): Doing the bislerp operation (see https://link.springer.com/article/10.1007/s10439-012-0593-5)

Additionally, this enhances the README with many needed info for a quick start.

[davidrudlstorfer]

Actually just a lot of questions/ideas on the coding style.

If the tests of the implemented FE methods lead to the correct results I hope that the implementation is correct as well :D

[davidrudlstorfer]

Really nice changes 😍

The src layout is pretty nice and I really like the slimmed down and uniform docstrings. Also really nice, that a lot of the basic functionality was outsourced to symfem. And last but not least, the vtu files look much nicer if they are readable. Nice improvements!