Payment Fee; add calculation function to finPaymentMethod

The code to calculate a Payment Fee has been centralised to a function on finPaymentMethod - where the fee is defined.

The new function is FeeCalculate and returns the Fee value. It has the following parameters

Parameters:

• value
• The Payment (before the Fee is added). A negative indicates a Withdrawal.
• usedPercentageFee (optional)
• Returns True if a percentage was used to calculate the Fee.

Notes:

• For a Payment the Percentage Fee is treated as a grossed-up calculation.
• E.g. 5% on $100.00 is $5.26 - 5% of $105.26 is $5.26.
• For a Withdrawal the Percentage Fee is treated as a normal calculation.
• E.g. 5% on $100.00 is $5.00.