Add lscv bandwidth selector for univariate kde

This bandwidth selection function use the method in page 61 to 66 in B. W. Silverman's book _Density Estimation for Statistics and Data Analysis_. This method works for symmetric kernels only. 

There are some errors in the book and I correct them in the code with my understandings. The estimated bandwidth match that estimated by `bwlscv` function from `Nonprametric`.