Implicitely call freeTransform in all functions that modify the problem.

[rschwarz]

Up to now, we called SCIPfreeTransform directly after SCIPsolve, but stored (copied) some important results, such as best solution, objective value and status.

We now want to expose more results (such as solving time, see #18 in SCIP.jl) and this approach is no longer viable.

First, we should remove the call to SCIPfreeTransform in the solve, but introduce it in all modifying methods.

Then (optionally) clean up some of the copied results that we no longer need.