Naive Bayes should provide better argument checking for negative symbols

[cesarsouza]

Negative symbols may arise when using a Codification filter to obtain integer symbols from a dataset that contains missing values, as exemplified in:

• https://stackoverflow.com/questions/45823479/accord-net-naivebayeslearning-index-was-outside-the-bounds-of-the-array/45824447?noredirect=1#comment78697686_45824447

The framework should offer more descriptive error messages and point to one of the examples on how missing values can be handled.

[cesarsouza]

Fixed in 3.8.0.