PubChem updates: some fixes, missing unit tests, and a single new API method

Hi Arvid,

we're about the submit a paper on PubChem where I hope to squeeze in some Bioclipse use case. That will require an added method, to allow this BSL:

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store = rdf.createInMemoryStore()
pubchem.downloadRDF(404, store)

rdf.addObjectProperty(
  store,
  "http://www.bioclipse.org/PrimaryObject",
  "http://www.bioclipse.org/hasURI",
  "http://http://rdf.ncbi.nlm.nih.gov/pubchem/compound/CID404"
)

props = isbjørn.getProperties(store)
props.each { prop ->
  println prop
}
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The seven commits are actually not about the two new API methods (and I stress that no API has changed). The new code is introduced in commits 3,4 and 7, while commit 1,2,5,6 are fixes, addressing:

- missing unit tests (commit 1,2,5)
- incorrect JavaDoc (commit 6)