Implement continuum Landau level calculation

Introduce machinery to compute Landau levels of continuum Hamiltonians. Continuation of !172 (closed), which introduces the core functionality to compute the Landau levels in the harmonic oscillator basis.

The core functionality is directly copied from !172 (closed) (excluding some minor fixes), with the exception of the combine function, which has been extended here to also pass on default argument values to the returned function when appropriate. The other new features introduced here are an interface between the Landau level code and Kwant, which makes using the Landau level code easier.

This is a rewrite from the ground up that produces a bona-fide Builder