Describe the bug

QCElemental allows floating point numbers for molecular charges, however when computing the spin with the fractional electron there is no way to represent the molecular multiplicity with just an integer.

To Reproduce

import qcelemental as qcel
mol = qcel.models.Molecule(
    geometry=[0.0, 0.0, 0.5, 0.0, 0.0, -0.5],
    symbols=["H", "He"],
    molecular_charge=0.5,
    molecular_multiplicity=1.5,  # not possible
)

Expected behavior

Consistent behavior for specifying molecular charge and molecular multiplicity.

Additional context