SQUASHME: minor fixes

- in benchmarks/cache.ipynb:
  - rename runcom -> rc (my bad!)
  - lower dimensionality from logspace(1, 4) to logspace(1, 3)
    (these were taking super long, and populations with >1000 dimensions
    are pretty rare)
- made sure Cache.get_files doesn't crash if legacy cache files exist
  (I had a crash because .cache/nengo/decoders/HASH.pkl isn't a
  directory)
- added _remove_legacy_files function so that people who used the cache
  before the subfolders change don't have a bunch of files lying around
  for no reason
- a few fixes in `test_cache` that were still assuming filenames from
  Cache.get_files() rather than paths.
- use () instead of line continuation character in test_cache assert
- use relative import in utils/nco.py and add it to utils/__init__
  (as well as some other files that should have already been added)
- Fixed a few typos / compressed text in docstrings
- It was unclear to me in nco.read whether we should be passing in
  the already pickled data, or the object to be pickled. Renamed
  pickle_data to metadata to make it clear that it's the object,
  not the pickled data.