Parse test data only once

[pcolby]

To make the tests run faster (they are taking quite a long time to run now), it should be possible to parse all session data once (in TestTrainingSession::initTestCase) to immutable objects, then re-use those (making mutable copies where necessary) in all tests.

This would require:

• an appropriate copy constructor
• a number of TrainingSession methods to be marked as const (which they should have been already).

[pcolby]

Same goes for the XSD files too. Though they don't get parsed all that many times, and won't improve the speed by much.