LumiX 0.2.0

Added

• keys= on LXLinearExpression.add_term, add_multi_term and sum_over: select the
  instances of a term by index key instead of a where filter. Builds the same model,
  much faster for large variable families.
• LXKeyFilter, the callable that stores keys= (it works anywhere a where works).
• LXVariable.index_key(instance): the key adapters use for an instance.
• User guide: Performance Considerations.

Changed

• Solver adapters fetch each variable family's instances once per build instead of once
  per constraint row. ORM-backed families are queried once per build. The cache is cleared
  on every build.
• Solver adapters no longer re-check terms whose instances were already selected by key.

Performance

• Transportation model with 600 rows and 90,000 variables (OR-Tools): build time 4.78 s in
  0.1.2; 2.88 s with where and 0.43 s with keys= in 0.2.0.