Fill the d orbitals

You will write the three steps that fill the d orbitals of an octahedral complex: decide whether the field is strong, split the electrons between the $t_{2g}$ and $e_g$ sets, and find the CFSE.

The idea. If $\Delta_o > P$ the complex is low spin and the $t_{2g}$ set fills first, up to six electrons. If not, each of the five orbitals takes one electron before any pairing. $\text{CFSE} = (-0.4\,n_{t_{2g}} + 0.6\,n_{e_g})\,\Delta_o$.

Your tasks.

1. Complete is_low_spin(delta, pairing) so that it returns True when the splitting is larger than the pairing energy.
2. Complete fill(d_count, low_spin) so that it returns a pair (t2g, eg), the numbers of electrons in the two sets.
3. Complete cfse(t2g, eg) so that it returns the CFSE in units of $\Delta_o$.