Re-introduce optional ks, or make new function with optional ks

Currently our tb_to_kgrid function does not allow the user to specify the k-grid, and only takes the number of k-points. This function is used both for the main algorithm, where it absolutely needs a regular grid in order for everything to work, and this function is also frequently used by the user (when plotting eigenvalues, calculating gaps etc.). The user will likely want to use this function also on a non-regular grid, so that for example they can zoom into the gap with more precision. Thus we should either reintroduce an optional k-grid to the tb_to_kgrid or create a new function which does the exact same thing as this current function but which does accept a non-regular k-grid.