feat(units): Add rpm unit

[adrfantini]

This PR adds RPM, the standard unit for engine revolutions, to the list of accepted units.

I considered also adding RPS (revolutions per seconds) but I think people could use Hz for that? They do in the cold storage (e.g. fridge compressors) sector.

[gwhitney]

Sorry, mathjs uses a system in which angle is a fundamental dimension of units, coequal with mass, length, etc. In such a system in seems certain to me that 1 rpm should equal τ/60 rad/s. Do you agree and would you like to revise your PR accordingly before Jos considers it? (I don't see it as being accepted in its current form, in which rpm is defined just as 1/60 Hz, ignoring the rotations/revolutions part. Speaking of which, there is already a cycle unit equal to τ rad, should there also be a revolution or rotation unit also equal to τ rad? )

And as with "Ah" #3617, this PR would need to update documentation and tests as well. Thanks so much!

[adrfantini]

Sorry, mathjs uses a system in which angle is a fundamental dimension of units, coequal with mass, length, etc. In such a system in seems certain to me that 1 rpm should equal τ/60 rad/s. Do you agree and would you like to revise your PR accordingly before Jos considers it? (I don't see it as being accepted in its current form, in which rpm is defined just as 1/60 Hz, ignoring the rotations/revolutions part. Speaking of which, there is already a cycle unit equal to τ rad, should there also be a revolution or rotation unit also equal to τ rad? )

And as with "Ah" #3617, this PR would need to update documentation and tests as well. Thanks so much!

ah, my bad. Of course, I'll revise it accordingly ASAP