Functions to calculate the HF of the user based on its position.

• supply does not force HF calc because it always increases it
• withdraw enforces HF calc
• borrow enforces HF calc
• repay does not force HF calc (if not repaying with aTokens)

We must evaluate if we want to change HF presentation: using percentage instead of bps. We should aim for simplicity because conversations can be made at UI level.