dict-like LRU for DataFrames backed by on-disk feather format
files
>>> from awj import AWJ
>>> awj = AWJ('/tmp/df_cache', max_size=500)
>>> awj['a'] = dfA
>>> awj['b'] = dfB
>>> list(awj)
['a', 'b']
>>> del awj['a']
>>> 'b' in awj
True
>>> 'a' in awj
False
If the on-disk size of the cache gets above max_sizeMB the least
recently used files are deleted until the cache is under size. The
first insert before pruning will save to disk before the pruning is
done, the max_size is more of a suggestion than a guarantee.
Keys must be strings which can be used as part of a filename.
The full dict interface has is not implemented (yet).
There are feathers poking out everywhere.