Address logic duplication related to health factor calculation and BalanceSheet construction

[jordanschalm]

Consolidate balance sheet construction logic with health factor computation. See comments in #77 related to:

• maxWithdraw
• _getUpdatedBalanceSheet should accept PositionView instead of pid
• positionHealth

[jordanschalm]

Closing as wont-do -- the duplicated logic is using internal vs external types -- not worth the effort right now.