Add Mass / MassFlow => TimeSpan operator.

[andrewimcclement]

This implements a missing operator (when compared with e.g. Volume, which has Volume / VolumeFlow => TimeSpan).

[lipchev]

@angularsen Note: we've already implemented this in v6 using the UnitRelations.json. Here's the most recent test for it:

        [Theory]
        [InlineData(10, MassUnit.Kilogram,
            5, MassFlowUnit.KilogramPerSecond,
            2, DurationUnit.Second)]
        public void Dividing_Mass_By_MassFlow_ReturnsDuration(double massValue, MassUnit massUnit, double massFlowValue,
            MassFlowUnit massFlowUnit, double expectedDurationValue, DurationUnit expectedDurationUnit)
        {
            var mass = new Mass(massValue, massUnit);
            var massFlow = new MassFlow(massFlowValue, massFlowUnit);
            var expectedDuration = new Duration(expectedDurationValue, expectedDurationUnit);

            Duration duration = mass / massFlow;

            Assert.Equal(expectedDuration, duration);
        }

[andrewimcclement]

See https://github.com/angularsen/UnitsNet/blob/release/v6/Common/UnitRelations.json for the file @lipchev is referring to.
Great to see it's already implemented. Let me know if you want this PR abandoned or not.

[andrewimcclement]

Abandoning on basis that it is implemented in v6.