Normalize unit representations

[wdduncan]

In the data we see many representations for units. E.g.,

7.0grams 7 g/L 7.0 grams per liter

We need to standardize into form of:

1. {float} {unit}
2. spellings and abbreviations

Also, as an add on, we can add some conversion logic to get everything into the same unit measurments.

cc @cmungall @realmarcin @hrshdhgd