Fix checksums.

If data checksum generation fails, return nan (as nan != nan).
Add checksum to domain, currently equals to its hash.
Optimize redraws when input signals change.
Fix drawing statistics when attribute order changes.
Negate selection when filter.negate is selected.

Closes #56
Add __str__ to FilterContinuous.
Moved gmm implementation from archive/gmm.
Display groups on widget.
Code style.
Set number of optimization steps as a parameter.
Fix crash when one cluster lost all members.
Fix scaling problem.
Seed clusters with kmeans.