Update MICM wrapper for species unit conversions

[boulderdaze]

Closes #73

• Added two unit conversion functions i.e. mol m-3 -> kg kg-1 and vise versa
• Updated the meta data
• Set up the test environment within the repository

[nusbaume]

@boulderdaze Looks good! Just have some hopefully minor change requests.

Also I am happy to leave the final decision up to you, @mattldawson, and others, but I personally think we could incorporate the Github Action test you made in your fork into this repo. I did request some changes that might require modifying the test itself, and its unclear what our final testing framework will look like, but all else being equal I would rather get some test coverage in now then wait for a "perfect" system later. If you all agree then we can either add the test and Github action to this PR now or add it as a separate PR later (either way is fine with me). Thanks!

[nusbaume]

I think it would be pretty easy to do this check in the constituents object itself during instantiation, or during the "set" call, as having a molar mass of zero is unphysical. What do you think @peverwhee? If so then maybe we can get this done sooner rather than later.

[nusbaume]

Thanks for the new testing infrastructure @boulderdaze! I did have some more questions and suggestions, but hopefully nothing that causes any major difficulties.

[mattldawson]

looks great! I think having these tests started is going to be very helpful moving forward

[nusbaume]

Everything looks great to me now, thanks again!

[peverwhee]

Thanks Jiwon! A couple of small comments, but nothing that should hold up the PR.