[margin] One-Direction, Two-Direction, Net-Out

One-Direction: newly opened position has to be in the same direction (buy or sell) as currently open position

Two-Direction: newly opened position does not have to be in the same direction (buy or sell) as currently open position

Net-Out: newly opened position cancels out currently open position. For example, one long position for 3 BTC and one short position for 2 BTC opened after in Net-Out mode will result in one long position for 1 BTC.