Change CTS to use REST API

[Aariq]

While puzzling through some problems with cts_*() functions (see #255, #243 ) I noticed that CTS has a REST API that is not currently implemented in the cts_*() functions.
https://cts.fiehnlab.ucdavis.edu/services

[Aariq]

The REST API currently seems broken, but I'm watching an issue and will take a crack at this when the API is working again. https://bitbucket.org/fiehnlab/ctsproxy/issues/24/ssl-certificate-has-expired-for-rest-api

[stitam]

Could httr::RETRY() mitigate our difficulties with the cts functions? If yes, then this may be a simple fix, I am happy to submit it.

[Aariq]

If it's an easy change, then it might be a good stopgap measure until CTS gets fixed.

[stitam]

Ok, then I'll jump in on this one.

[stitam]

I am closing this issue as REST API would still be nice, but CTS seems to be working fine so I'll just mark it as low priority.

[Aariq]

REST API seems to be working now! Features available by REST that are not currently in webchem::cts_* functions:

• Convert from chemical name to InChiKey with either a "popularity" or "biological" (?) score—could be integrated with the match = argument for cts_convert().
• Convert InChiKey to MOL/SDF
• Count biological IDs. I think this means 'how many hits are there for this InChiKey in various biological molecule databases?' Only works with InChiKey as input.