Obtaining and storing the LP solution

[Peter Kovacs]

It would be nice if the LP and MIP solver interfaces provided functions for obtaining the LP solution at once. We could use a class that can store the solution efficiently, i.e. it should store variable-value pairs for the non-zero valued variables.

This ticket is a follow-up of #326.

[Peter Kovacs]

Two possiblities were discussed in #326 (see details and reasons there).

1. We could use the Expr type for this purpose, since its implementation is suitable for that (it stores column-value pairs). However, it would be strange, because the name of the class and its members reflects the other purpose for which the class can be used. Now it is used only for storing linear expressions, i.e. certain coefficients are assigned to the variables, and all namings correspond to this usage.

2. We could introduce another class, e.g. Solution for this purpose, which could use the same or similar implementation as Expr, but its members would have names that are more suitable for this purpose (e.g. value instead of coefficient). This class could be a wrapper for Expr or have a separate implementation. However, both ways would make the code maintenance harder.

Of course, DualExpr or DualSolution could be used for stroing the dual solution.

Another important question is the names of the functions that can be used to obtain the primal/dual solution.