Added unassigned lectures to Solution

Solution now has a TreeSet of unassigned lectures, will make generation
of complete solutions easier
Revert "Added unassigned lectures to Solution"

This reverts commit 2dd04c2998bedc25d6595aed4734144fd7d6c13f.
Revert "Revert "Added unassigned lectures to Solution""

This reverts commit 5b5ee23ba9b0373f4efe14a79a667bb891f8f7be.
Oops

Forgot getter method