nonparametric examples and some fixes

rebased on current master, after PR #562 merge

- mostly example script files to check how well kernel regression works
- reduce warnings in doc build, remove Latex math errors  (raw string instead of double backslash)
- KernelCensoredReg  bug? add W  see also issue #600
- fixes in SingleIndex and SemiLinear  see also issues  #605 #607

there is stilll work to do on nonparametric kernel methods, 
I mainly looked at kernel regression classes in detail.