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`. 

There are two differences. 
The first is the constant term `ba²/2pi * c`, only `ba` is used in his book and that is an error. The wrong constant will give a wrong weight after adding `pdf(dist, 0.0)/n`. 
The second difference: `(cf^2-2cf)` is the general form while the special case of Gaussian is used in the book.