samplesize! for ratiobasedsampling

Changelog

• Dataloaders.samplesize! allows setting the total sample size,
• Random number generation for sampling in Dataloaders is controlled by local rng instead of seeding the global rng,
• skiperrors argument to functions Dataloaders.buildindex and Dataloaders.countlabels,
• MWT.interpolate supports behavior labels in the shape of String, Symbol and Vector thereof.