Merge pull request #1240 from josef-pkt/kde_weights_1103_823

Kde weights closes #1103 closes #823 closes #1245
Fixed bug in uniform kernel (returned a scalar instead of array; made KDEUnivariate fail).
BUG: fix kernels smoothconf standard error, rename Biweight.smoothconf see #1231
TST: kernels smoothconf against Stata, confint is only approx. see #1231
TST: adjust precision kernel.smoothconf
TST: keep disabled cosine2 of Stata, fitted matches, strange se