Add missing SMARTS strings and correctness tests for `get_fx_groups()`

[Aariq]

Addresses #44 (partially) and #47

Adds SMARTS strings to capture all functional groups in the SIMPOL.1 paper except "number of carbons on the acid side of an amide" which I think is probably impossible with SMARTS alone. Adds a CSV file with test compounds and their expected functional group counts as well as a test to compare to that CSV.

[Aariq]

Still left to figure out:

• carbon number on the acid-side of amide (tricky, maybe impossible? maybe just need to understand $ operator better?)
• carbonylperoxynitrate (haven't tried yet)
• nitrophenol (tricky because of definition—aromatic OH when there's also a nitro group anywhere on the ring)
• nitroesther (confused about definition)

[Aariq]

I think carbon number on the acid-side of amide is not possible with just SMARTS. Would need to chop the bond on the acid side of the amide, then count carbons in the remaining piece and I don't think that kind of sub-setting is something SMARTS can do.

[Aariq]

I think I'll want to "Squash and Merge" this one

[Aariq]

I noticed that in the KEGG vignette phosphatidylcholine moved up to "high" volatility with this PR and that just can't be right. Need to investigate what changed for this compound.

[Aariq]

Still need to:

• [x] double-check that all the functional groups have tests (i.e. the colnames in test_compounds.csv match)
• [x] make notes on test_compounds.csv about why each compound was chosen
• [x] add example compounds for secondary and tertiary amines and aldehydes
• [x] add example compounds for:
  • phosphoric_acid
  • phosphoric_ester
  • sulfate
  • sulfonate
  • thiol
  • carbothioester