Fix conversions between rpm, rps, and other rotational units.

[ajkeller34]

Closes #261.

[codecov-io]

Codecov Report

Merging #268 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #268      +/-   ##
==========================================
+ Coverage   78.32%   78.34%   +0.02%     
==========================================
  Files          15       15              
  Lines        1075     1076       +1     
==========================================
+ Hits          842      843       +1     
  Misses        233      233

Impacted Files	Coverage Δ
src/pkgdefaults.jl	20.4% <ø> (ø)	⬆️
src/user.jl	94.69% <0%> (+0.04%)	⬆️

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[ajkeller34]

I'd like to keep this open for a couple days to see if there are any objections. In particular, I think the previous definition of Hz was also wrong. I'm surprised nobody complained about it before given how common of a unit Hz is, which is why I'm nervous to merge this.

[gustaphe]

I know I'm a bit late to the party but I do object to this. I would expect 60rpm == 1Hz == 1/s. To get an angular frequency (in radians/minute) corresponding to 1 revolution per minute, you take ω = 2π*f. This tripped me up bad recently.