support weight aes in stat_sample_slabinterval

[mjskay]

To make everything consistent this would be a bit of a pain, so leaving it off for now. Would need to:

• make a weighted_ecdf() function to support CDFs
• make a weighted_quantile() function to support quantiles
• make a weighted_hist() function to support histograms
• make a weighted sample distribution type (.dist_weighted_sample() for now)
• pull out weight aes and summarize sample into a .dist_weighted_sample() in compute_panel
• pass through weight to density, weighted_ecdf, and weighted_quantile for slabs
• use weighted histograms to support histinterval
• add support for weights in point_interval - at least for weighted sample dist objects

Later, once support in posterior is done:

• support weights in rvars