Add synonyms to search

[BenPortner]

In ecoinvent3, there is no activity PTFE production but when searching for PTFE in ecoquery, one finds an activity tetrafluoroethylene film production, on glass that seems to mean the same thing. Ecoquery find's it, because it includes synonyms in the search index. Brightway does not, hence the activity cannot be found. To improve search in brightway, I suggest parsing synonyms from the ecospold files and adding them to the search index. This requires changes in bw2io and bw2data:

• [x] extract synonyms from ecospold2 files (rewrite bw2io's ecospold2extractor)
• [x] add synonyms to the search index (rewrite bw2data's schema/indices/search.py)
• [x] add strategy to remove None synonyms from ecospold2 data
• [x] add tests for synonym extraction, search, and strategy
• [x] make sure all tests are running fine (both bw2io and bw2data)

[BenPortner]

Problem: At least one ecospold file (electricity production, natural gas, conventional power plant, TH) contains an empty synonym tag <synonym xml:lang="en" />. This causes an error when importing ecoinvent:

  File "C:\Users\benjamin.portner\programs\miniconda\envs\bw_dev\lib\site-packages\bw2data\search\indices.py", line 33, in _format_dataset
    synonyms=u", ".join(ds.get("synonyms", [])).lower(),
TypeError: sequence item 0: expected str instance, NoneType found

Proposed mitigation: Create a strategy to replace "synonyms": [None] with "synonyms": []