Add MassFlux

[gojanpaolo]

Can we add MassFlux (Mass/Area/Time) to the library?
Let me know if we can then I'll submit a PR when I get some free time.
Thank you! :)

[angularsen]

I don't see why not. I have no idea how widely used it is, but seems to be a well established quantity. 😀

[gojanpaolo]

Great! Will submit PR a bit later.
We usually encounter it when multiplying Density and Velocity which becomes MassFlux.
The MassFlux can then be used to get MassFlowRate given an area or used as a factor to calculate another Density or Velocity.