fix: add G(::AbstractDrive) dispatch to fix downstream MethodError

The LinearDrive/NonlinearDrive feature changed QuantumSystem.H_drives from
Vector{SparseMatrixCSC} to Vector{AbstractDrive}. Downstream code (e.g.
Piccolissimo's integrator) that relies on behavior of Isomorphisms, G,
and sys.H_drives.

This adds G(d::AbstractDrive) = G(d.H) so that G transparently delegates
to the underlying Hamiltonian matrix, restoring compatibility for any code
that calls G.(sys.H_drives).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
