Some exchanges (BitMEX) return integer values for Volume field. And sometimes even for OHLC -- same, on BitMEX, since price decrease is 0.5. TA-LIB functions assume floats and fail with exception.
Of course, this can be fixed (converted) in ccxt for particular exchange, but TA-LIB will still fail for exchanges for that such a conversion is not implemented in ccxt code. So let's make perform this conversion here in order to be sure our strategy will not crash on a new exchange.
