Add specific number rates: volumetric, areal, and linear.

[crystal-growth]

Add specific number rates

• Volumetric number rate [1 / (Volume x Time)]
• Areal number rate [1 / (Area x Time)]
• Linear number rate [1/ (Length x Time)]

Kind is ConstituentConcentrationKind

Typical applications:

• molecule collision rate
• carrier recombination rate in semiconductor (volumetric for bulk recombination, areal for surface recombination and linear for recombination in linear defect or quantum wire)
• nucleation rate (volumetric for homogeneous nucleation, areal for heterogeneous nucleation on surface)

[iliekturtles]

I added a bunch of additional units to this one. Please review. Any more that you think should be added? Once you think things look good you can squash my changes into your commit: git rebase -i origin/master --autosquash. Let me know when that is done and I can do a final review and merge.

[crystal-growth]

Thank you, I think now the set of units covers all possible practical problems. I've rebased and squashed the commit.

[iliekturtles]

Everything I've reviewed so far has been merged! Thank you again for all these PRs. I'm going to try to get to the rest next week.

[crystal-growth]

Thank you! I'll now delete merged branches.