Handle obsoletes, or provide option to ignore

[caufieldjh]

Semsimian should be aware of obsolete terms and ignore either by default or when specified.

An example from the Monarch API:
http://api-dev.monarchinitiative.org/v3/api/semsim/search/HP%3A0002104%2CHP%3A0012378%2CHP%3A0012378%2CHP%3A0012378/Human%20Diseases?limit=10

The top hit here is an obsolete.

[matentzn]

generally it would be good to define a clear SOP for obsoleted, including

1. what happens if they are in the query input set (should they be replaced by their replacedBy relation, if they exist?)
2. What happens if they are in the data (should they be rewired at ETL time)
3. What happens if they remain in the data and get returned to the user