Add macro to create register functions and move laplacian to single file

[luisfpereira]

This PR adds a macro to automatically create register_ functions, and moves everything that was inside laplacian folder to a single file (previous design was due to less robust registry; this way is simpler).

The register macro works as follow: given a Registry (e.g. LaplacianFinderRegistry) it creates a function register_ by removing Registry from the name and transforming it in snake case (e.g. register_laplacian_finder).
(This is a simple thing that may potentially make the code slightly harder to understand - you tell me @GiLonga and @gviga -, but it is annoying to create this functions every time).

[GiLonga]

I agree with you. Just yesterday, I was struggling with those lines of code myself. This new approach is more elegant but a bit tricky to understand at first glance. I think a more detailed docstring is necessary, something like:

Automatically create `register_` functions for each class registry in this module. 
These registers will be useful subsequently for operations in the Mixins classes. 
Example:
class LaplacianFinderRegistry(MeshWhichRegistry):
    MAP = {}
LaplacianFinderRegistry.register  -> this operation is now automatically performed by this function.

An example would help to clarify it.

[luisfpereira]

Spot on @GiLonga. I've updated the docstring (b61af64). Hope it makes it more clear. I think the example should be given within the Registry docstrings though. I'll update those soon for clarity.