Write C++ backend for margEff() continuous case

Closes #1
Move everything into main directory
Remove unused makeProfile() function
Update documentation for predict functions

Closes #5
Update dependencies and imports

Closes #2
Remove unused .rowProds() function
Split functions into own files

The exceptions are those in fn_poly.r and fitters.r, because they are
doomed anyway
Streamline plotting functions

* Remove "bag" argument (closes #7)
* Call predVals() directly (closes #4)
Use headers only for declarations, not definitions
Allow intercept to be excluded from poly expansion
Add function for cross-product of poly expansion
Add R frontend to computePolyTerms()
Implement coordinate descent algorithm for LASSO
Use a class for cross-product output