Add MassConcentration, MolarConcentration, and CatalyticActivityConcentration.

[okkn]

Hi. I added MassConcentration (ex. kg/m³, mg/dL), MolarConcentration (mol/m³, mol/L), and CatalyticActivityConcentration (kat/m³, U/L) to this fabulous library. To separate MassConcentration from MassDensity, I introduced a new Kind trait (ConstituentConcentrationKind).
These quantities are very often used in medicine and biology.
Regards,

[iliekturtles]

Thanks for the PR! I'm going to try to review tomorrow.

[iliekturtles]

Did a first pass review and only found a few extra new lines to remove. I still need to go back and review the units in more details.

[okkn]

As you indicated, these newlines are extra. I removed them.

[iliekturtles]

Found a couple more minor things. Go ahead and amend these commits into your last review commit.

Only thing I have left is to finish reviewing unit details for a couple of the files. Slow going, but given the attention to detail you have put in I'm not expecting to find much so we should be able to merge soon!

[iliekturtles]

One final line to remove shown in the diff below (ignore the encoding issues). If you could squash the fixes into the original commits I'll merge. Will likely look at the build timeout as a separate issue.

diff --git a/src/si/catalytic_activity_concentration.rs b/src/si/catalytic_activity_concentration.rs
index 638bb94..e44a1a3 100644
--- a/src/si/catalytic_activity_concentration.rs
+++ b/src/si/catalytic_activity_concentration.rs
@@ -4,7 +4,6 @@ quantity! {
     /// Catalytic activity concentration (base unit katal per cubic meter, mol · s⁻¹ · m⁻³).
     quantity: CatalyticActivityConcentration; "catalytic activity concentration";
     /// Dimension of catalytic activity concentration, L⁻³T⁻¹N (base unit katal, mol · s⁻¹ · m⁻³).
-    /// (base unit katal per cubic meter, mol · s⁻¹ · m⁻³).
     dimension: ISQ<
         N3,     // length
         Z0,     // mass

[okkn]

Fixed😉

[iliekturtles]

I squashed the fixes into the original commits and merged. Thanks so much for the PR!