Naming

[ChrisRackauckas]

There's a few naming things I think we should clear up.

PDEOperators.jl is a little off. There are many non-PDE differential equations for which you would want to use these. The classic example is a 2nd order BVP where the Laplacian shows up. I think we should move the whole AbstractDiffEqOperator stuff here (which is actually like 40 lines...) and just name it DiffEqOperators.jl since that covers the field better and matches the naming scheme anyways (everything has DiffEq in it somewhere).

The LinearOperator type is a little vague. It doesn't capture all or most linear operators, so it should probably be DerivativeOperator since all of these operators are discretizations of derivatives.

[ChrisRackauckas]

Implemented in #27