MipSolver interface fixes and extension

[Peter Kovacs]

In MipSolver there are type(), sol() and solValue() functions to obtain the (primal) solution.

1. There are two inconsistent names here:
   • solValue() should be renamed to sol() (as on overloaded version) as primalValue() was renamed to primal() in LpSolver.
   • ColTypes should be renamed to ColType, since we do not use plural in similar names.

2. Apart form that, I suggest an extension. Let's introduce primalType() and primal() as an alias for the above functions. Adding these variants, one can change to using a Mip solver instead of an Lp solver without rewriting the code (if only primal queries were used).

3. Maybe type(), sol() could also be added to LpSolver as an alias for the primal queries.