Dynamically get base turn duration for trade routes

[JamieNyanchi]

When calculating the duration of trade routes, dynamically get the base turn duration instead of using a hardcoded value of 20. Rewrote the formula slightly to divide iSpeedCostMultiplier by 100 instead of 10, as this variable is a percentage multiplier.

This allows BTS to show the correct values when using mods that change the base duration length. I use both BTS and a mod that reduces the base duration length, and this change seems to work perfect.

Let me know if you have any comments or questions!

[r3a1d3a1]

I noticed this does some extra modifications you can see here. I haven't verified but if you're familiar, can you confirm if GetInternationalYieldModifier indexing starts at 0?