Remove extra sig digs from IV and improve constants

[haydenshively]

Three things here:

• Changed IV decimals from 18 to 12. 12 guarantees no overflow in the Volatility library, as opposed to just a strong unlikelihood. And 18 was way too many fake sig digs anyway.
• Instead of clamping before multiplying by nSigma in BalanceSheet, we now do it afterwards. This will allow us to set nSigma to higher values without worrying about underflow.
• Related to that clamping: I rewrote the min/max values in terms of loan-to-value. imo much clearer what they mean now