`stats.mstats` potential improvements

[jorenham]

• spearmanr
• kendalltau
• pointbiserialr ?
• linregress
• theilslopes
• siegelslopes
• sen_seasonal_slopes ?
• ttest_1samp
• ttest_ind
• mannwhitneyu ?
• kruskal ?
• ks_1samp ?
• ks_2samp ?
• kstest ?
• trim, trima, trimr, trimboth, trimtail !
• trimmed_mean !
• tmean !
• tvar !
• tmin, tmax !
• tsem !
• moment, variation, skew, kurtosis !!
• describe !
• stde_median
• skewtest, kurtosistest
• normaltest
• f_oneway ?
• friedmanchisquare ?
• brunnermunzel ?
• hdquantiles ?
• hdmedian ?
• hdquantiles_sd ?
• trimmed_mean_ci ?
• mjci ?
• mquantiles_cimj ?
• median_cihs ?
• compare_medians_ms ?
• idealfourths ?
• rsh ?

Legend:

• "?": need to be checked to see if they're actually correct now (dtype and/or shape)
• "!": should be prioritized