Goals for 1.0

[DanielVandH]

I want to update this package to v1.0. The main things I want to do before updating to 1.0 are:

• Add support for inhomogeneous Neumann boundary conditions #32
• Linear problems #17
• Rework the BoundaryCondition interface #35
• Either remove or update the interpolant interface #36

[DanielVandH]

It would be nice to think about how I could possibly incorporate FiniteVolumeMethod1D.jl into here, or if I should.

I also still need to convert my 3D FVM code from Matlab into Julia, which could go here as well maybe?