Generalise Mode to be any eigenmode, not just frequency eigenmode

[sp94]

The Mode object returned by solve(solver, k, polarisation) could be generalised

Currently it is assumed to be a frequency eigenmode

Mode(k0::Vector, frequency::Number, data::Vector...)

We should rename frequency to eigval, data to eigvec, and add a label string

Mode(k0::Vector, eigval::Number, eigvec::Vector, label::String, ...)

solve(solver, k, polarisation) could return array of Mode(k0, eigval, eigvec, "Frequency")

wilson_eigen() could return array of Mode(k0, eigval, eigvec, "Wilson loop eigenvalue")

The label field could be used by eg plot_band_diagram to automatically label the y-axis