Feature: Stepwise Selection

[aravindhebbali]

Add a function blr_stepwise() for step wise selection of predictors. It should include the following arguments:

• model: a binary logistic regression model
• include: predictors to be included in the model at the beginning of the step wise process
• enter: significance level at which the predictor will enter the model
• retain: significance level at which the predictor will be retained in the model