Add decimal overloads to cTrader API

All the methods in cAlgo use double, none of them use decimal.

Sometimes I may need to do operations in which I do not want to lose precision. Sometimes there may be some workarounds I can do with rounding, arithmetic and modulo, but seeing as cTrader is a financial application, it would be good in general for there to be methods that use decimal, and likewise for decimal to be used behind the scenes for these methods.