Routine to compute the ground state projector using the KPM. Note that this calculation
is done using sparse arrays, since it results in faster computation.
Parameters:
nmoments (int) – Number of moments in the expansion.
h (ndarray) – Bloch Hamiltonian, usually at k=0 for supercells.
energy (int) – Energy up to which the states are considered. To compute the ground state
projector, this should be the Fermi energy.
