Implements a class nbclassifier with necessary functions to implement Naive bayes classification.

Required arguments to initialise an nbclassifier object are:

data - text data to perform classification on. First column should contain labels and Second column should contain text data

method - legal values 'tfidf' for term frequency inverse document frequency, and 'ngram' for bag of words

n = number of words in an n-gram

split = split size of training data