coin-or/rbfopt: Stable v4.2.0

Added:

• Support for categorical variables (variable type 'C', in addition to 'R' for real and 'I' for integer). These variables are treated differently from integer variables, but they still need to have lower and upper bounds; e.g., a categorical variable with bounds [0, 2] can take on 3 possible values: 0, 1, 2.

• Renamed parameter names starting with 'tr' to 'ref', for better consistency with the literature.