Add molar unit

[jlumpe]

This is a really important one in chemistry and biology, I feel it should be included by default (especially since molarity is already included as a derived dimension).

[ajkeller34]

I agree and I'm happy to include this, but Molar has to be defined after Liter given the definition. I'll merge this once you make that change.

[codecov-io]

Codecov Report

Merging #258 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #258   +/-   ##
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  Coverage   78.43%   78.43%           
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  Files          15       15           
  Lines        1071     1071           
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  Hits          840      840           
  Misses        231      231

Impacted Files	Coverage Δ
src/pkgdefaults.jl	21.27% <ø> (ø)	⬆️

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[jlumpe]

Oops, fixed. I don't know if there are any tests I should incorporate it into, I didn't see any.

I'm also interested in adding a "count" unit with dimension 𝐍 which is equal to 1/6.022e23 of a mole, but I'm not really sure what its proper name or symbol should be. This would be very useful for converting from bulk (per mole) measurements to measurements per particle, like grams -> Daltons or in my case converting from concentration in a small volume like a cell to the actual number of molecules.

[ajkeller34]

Thanks for the PR. I'm hesitant to include a unit that doesn't have an established name or symbol, though I can see the utility. Of course you can easily define units in your own packages.

[erlebach]

I tried the following with Unitful installed:

julia> 1u"ml" / 1u"L"
1.0 mL L

I am surprised that you do not convert all units to SI and then the results would have been 10^{-3}. What is the rationale for not doing so? Thanks.