Density floor setting has inconsistent name in neutral_mixed

[mikekryjak]

Density floor is named density_floor in most places:

hermes-3/src/evolve_density.cxx

Line 30 in a7b552d

density_floor = options["density_floor"].doc("Minimum density floor").withDefault(1e-5);

But in neutral_mixed it's called nn_floor:

hermes-3/src/neutral_mixed.cxx

Lines 56 to 59 in a7b552d

	nn_floor = options["nn_floor"]
	.doc("A minimum density used when dividing NVn by Nn. "
	"Normalised units.")
	.withDefault(1e-5);

The name should either be the same, or the neutral_mixed name should be changed to a more distinct one.