Make R formula interface

[osorensen]

Overview

Start by defining an R interface for calling the galamm function with certain arguments, and from there generate the sparse matrices, mappings between parameters and sparse matrix entries, model family, and other things necessary to send to C++.

Intended outcome

Should be able to define models to be fitted from R.

Things to do

• Define model interface #7
• Model setup #8
• Define test data #9
• Unit tests for model setup #10