Revert "RegFit.R"

This reverts commit f05ca7b8195b362618d200f8e155e2d23171ed6b.
Finalize RegFit.R

RegFit.R file sets S4 class "RegFit", which is a subclass of
"RegStats".
Updates to RegFit_class.R + Change the class file names

Changed "RegStats.R" and "RegFit.R" to "RegStats_class.R" and
"RegFit_class.R".
Last line of setMethod is added to the RegFit class file.
returnImportance.R

This R file contains a function that evaluates the importance of the
coefficients. Specifically, it returns t-statistics and p-values of
each coefficient in each combination of regression.