Calculate correct amounts and prices for orders

[MarkoPaasila]

DEX orders can be always accurately expressed as fractional numbers, not decimal numbers. Strategies should calculate the exact fractional price and amounts that are actually possible, and use those values in the logic. They can be represented accurately enough using floats, but not every float value is possible. Placing orders that result in something else than expected causes hard to understand situations, and can be hard to debug. I suggest a method that, given a target price and amount, can return the closest actually possible values - which then can be further used in strategie's logic's.

[bitphage]

I proposed to have a method in the library to place an order with exact BASE and QUOTE amounts here
bitshares/python-bitshares#227

I think this solution would be cleaner than having special rounding method as in #512, as python-bithsares currently does own rounding in Market.sell() / Market.buy(), which means twice rounding.

[MarkoPaasila]

You still have to figure out what the exact amounts are. #512 gives you a method to find them using the price as decimal/float - like people normally think about prices - and not suffer from rounding. Your proposed method is what 512 was missing to make it perfect. Should I change 512 to also return/accept internal amounts? They are integers of course.