Add option to analyze error of state variables wrt analytical solution

We add two new options

• analyze_error: this computes u_err = u - u_ana, where u_ana is the analytical solution which needs to be implemented on a per-problem basis; so far we only implemented it for Advection and sine initial data
• analyze_error_norm: computes the L^2 norm of u_err; this would be 0d output, which is not yet implemented, so we don't test it yet.

This PR also includes a bit of a refactoring of setup.jl to clarify the register! mess (we done that already for other projects).