Added two new decoder optimization routines.

- they may be faster in some circumstances
Renamed pytest `benchmark` option to `benchmarks`
Added tests for new optimization routines.
Added heuristic to choose the number of eval points.

- fixes #290
Added benchmarks for number of eval points
Added check for empty activities matrix
Added tests for decoder solvers.

- also fixed bug with size of `lstsq_L1` output
Suppressed Python 3 flake8 errors.
Added `block_conjgrad` test.