Add support for Frequency Weights For Decomposition

[s3alfisc]

Allow for (frequency) weights to scale to very big data sets.

• Implement WLS estimator.
• Adjust bootstrap logic: fweights - use multinomial for resampling & re-weighting; aweights - simple rowwise resampling of weighted data

[s3alfisc]

Fweights (no inference) implemented with #1021.