Enforce a minimum balance per position

[vishalchangrani]

The protocol should enforce a minimum balance for every position, with the following behavior:

A minimum balance must be maintained after every deposit and withdrawal

• The check should be enforced on the position balance itself, not just on creation
• If an action would cause the position to fall below the minimum, it should revert
• This applies to all token / collateral types
• The minimum can initially be very simple (e.g. 1 unit of the reference value)
• Closing a position is exempt from this requirement
• The rebalancer does not need to account for this yet