Gauge covariant derivatives

[lxvm]

The goal of this pr will be to implement gauge-covariant derivative operators.
I explain the theory in the following memo:
gauge.pdf.
Given coefficients O_R for a Wannier interpolant of an operator O, the only additional data that is required for these derivatives is a Berry connection A_R.

What I would like to achieve is:

• gauge-covariant derivatives of arbitrary order (based on DerivativeSeries in FourierSeriesEvaluators.jl) and a choice to transform between Wannier and Hamiltonian gauges
• only make a distinction between operators that commute with the Hamiltonian (which defines the gauge that diagonalizes them) and those that don't
• an interface to compute derivatives of eigenvalues from the gauge-covariant derivatives (e.g. a generalization of the current Intra/Inter-band velocities) and a perturbation-theory calculation of the Berry connection in the Hamiltonian gauge (see the memo for how to do this for the inverse effective mass)
• refactoring AbstractWannierInterp to contain properties such as gauge and coord in a CommonSolve.jl problem
• Simplifying load_wannier90_data's interp keyword so that it tries to load all possible input files into multiple Fourier series, and later pick those needed for the physical observable requested by the problem
• some robust numerical test that an operator is in fact gauge-covariant (possibly based on symmetry)

It would be great to identify a unifying idea around which to organize the implementation, such as the gauge-covariant derivative mentioned above, as the current status of src/interp.jl is just a mish-mash of AbstractFourierSeries constructed from one another to obtain the requested observable.