Bugfix: calculate Mykobo fee correctly

[TorstenStueber]

Currently we define a 5% fee for Mykobo in order to calculate the final amount the user will receive. This is incorrect.

TODO

• the actual fee is 1.25% instead
• currently the absolute fee is calculated by rounding down, instead it needs to be rounded to the closest cent value (e.g., the fee for an input value of 1000.4 is 12.51, not 12.5).

[vadaynujra]

@TorstenStueber For the implementation of this fix, we should do it in a way that is not fixed for all off ramping currencies but different for each as each anchor that is off ramping the asset is expected to have a different fee.

[TorstenStueber]

The fee is not hardcoded but defined by anchor asset.

For other anchors we might need to extend the fee calculation as they might use a combination of fixed and relative fees.