[FEATURE] Update survey score objects to have decimal scores

[akshadpai]

Currently, the score class in the surveys has the score represented as an int. We want to change this value to a double.

We also need to add a parameter 'correctAnswerCount' from survey stats and compute it for the building block.

The score should also store a parameter 'difficulty' to keep track of what mode the user last played.

This is related to the following issue for Vogue #2051.