Make params.txt an input file

It is more intuitive to modify an input file than to hardcode values into main.cpp. Rather than having main.cpp spit out the hardcoded values, it would be nice if the only file getting modified is the input values (which would be params.txt or a renamed version). This way, the code wouldn't need to be recompiled when input params are changed!

Start by reading main.cpp and the existing params.txt to understand which values are hardcoded and how the input file is structured. Trace where the model currently receives those values, then verify that changing params.txt changes the run without recompiling main.cpp.

Written by the indexing model from the issue text.