[Feature Request] weighted KNN imputation

FSharp.Stats already supports the KNN imputation via `FSharp.Stats.ML.Impute.kNearestImpute`. The current implementation takes the k nearest neighbors and computes the average of these at the index of interest. This average replaces the missing value of the incomplete data point. I suggest to make the following changes/additions:

- rename the module to `Imputation` to be consistent within the library
- add the possibility to define how a missing value is encoded (e.g., 0.0 or nan)
- add an optional converter function that processes the distance measure. When using Pearson's correlation coefficient you determine the similarity rather the distance and therefore you have to take the reciprocal.
- add a weighted version in which the averaging can be weighted according to the distance of the nearest neighbors
- add proper documentation

## Keywords

- Local Least Squares