Renaming prior to registration

[maltezfaria]

I think a few (or many, we will see) structures and functions could use a better name in Inti. I am opening this issue so that we can start collecting and discussing them. Once we agree, we can do a single "search and replace" to PR to fix the issues.

@caperezar and @tanderson92 feel free to edit this list. Here is what I have so far:

• LagrangeMesh --> Mesh: poor choice since this mesh can support other types of elements (e.g. ParametricElement)
• domain --> reference_shape: for a ReferenceInterpolant, the domain method is used to return its ReferenceShape. I think it would be more clear to rename it
• Quadrature --> MeshQuadrature: I'm not so sure about this one, but our Quadrature structure is quite rich since it contains the mesh and metadata to index using domains
• Compression methods --> acceleration methods?
• AbstractPDE --> AbstractDifferentialOperator, and rename the keyword pde to operator in the various API methods.