Fixed resource choice tests, removed stale student tests.
Student class changes too often - it's central element in the whole simulation, so it's critical to keep it correct, but on the other hand the tendency was to move pieces of functionality out of it.
On the other hand student tests already had low coverage, so they add little value, while draw much attention. Thus I decided to hold on with those Student tests until Student class stabilized.
Improved lesson search by type in resource and curriculum
Added PassExamGoal
Refactored PassExamActivity so that exam is chosen as part of activity:
* Added weight to exams
* Added student methods for choosing the exam
* Rewritten PassExamActivity so that exam choice happens inside it
Improved studying - student does not study lectures that have no new facts.