[BUG] Velocity removal check is invalid if molecule has perturbable and non-perturbable properties

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We currently remove velocity properties when not all molecules in the system have them, e.g. when combining molecules. The check makes sure that the total number of velocity properties (non-perturbable and perturbable) is not-equal to the number of molecules in the system. However, it is possible for a molecule to have both types of property, so we should instead check that the total number is not less than the number of molecules.