Harmonization of substances

[tomlue]

After integrating three databases in #1 we need to start harmonizing things. Lets begin with chemicals. One process is to:

1. Reshape the substances table to SID, KEY, VALUE
2. Match substance ids by those that share the same identifiers
3. look for substances where some identifiers match, but others don't. These require conflict resolution (later step)

Example:

ksjdlk-ksdfj-ksdjf {cas:"123-1243-1233"}
qweret-qwer-ksdjf {cas:"123-1243-1233"}

becomes

ksjdlk-ksdfj-ksdjf cas 123-1243-1233
qweret-qwer-ksdjf cas 123-1243-1233

and we can match row 1 and 2 because they share a cas number

Note*
We might want to add an additional column to the substance table identifying the source(s) of the substance. This might allow us to improve chemical matching and curation. For example, one source might use the key 'casrn' to identify the cas number while another resource might use 'cas'. In general, we should try to normalize the keys used by different resources, and knowing which sources use what is one step in that process.