Grid bot: percentage-based step size

Please add the possibility to choose arithmetic or geometric mode for ordering orders.

Arithmetic is now used. Step between orders to always the same in absolute value.

Geometric has a step as a percentage and the percentage profit is always the same.

Geometric is good for GRID with a wide range of price. When the price changes by 50-100%, the arithmetic has too large, or a small step between orders (depending on which side the price moves).