WIP: building smaller thesauri on demand
Fixed failing unit test from yesterday
Moved allow_lexical_overlap to a field
Vectors.get_nearest_neighbours removing self by default

This matches Byblo's behaviour. Also added a thesaurus built with byblo
and a test to compare it to the neighbours I generate on the fly in
Vectors
Picklable Delayed
Parameters now passed into Vectors constructor by from_shelf_readonly

Previously allow_lexical_overlap was effectively ignored when calling
from_shelf, resulting in different neighbours when using the optional
shelving step