RS and GRS should be merged into one

[David Lucas]

For now, there's a rs and a grs file. These two should be merged, with column multipliers as an optional argument. If nothing is set as col_mult, it's an RS code (default value 1 for all col_mults), else it's a GRS code.

[Johan Rosenkilde]

The optional argument could be called column_multipliers for verbose clarity. I think that's the general naming ideology of Python and Sage.