Make startLearning() throw an exception on repeated invocations

[mtf90]

To enforce the contract of the documentation of startLearning(), the code should throw an IllegalStateException if the method is invoked twice.

With the recent support of access sequence transformations (which require the learning process to have started), the same logic can be used to check that startLearning is not called in the same learning state.

Checking this can be easily integrated into existing integration tests.