initial commit

[ColinRhys]

For issue - "Allow user to specify a fee rate"

• Allow user to specify a fee rate #3

Code Changes:

• Added optional parameter user_fee_rate for calculate_fee function
• If / else block checking for user_fee_rate
• Use user_fee_rate if present
• Added test for specific scenario of a user passing in user_fee_rate. The test checks individual rate variables in the ProjectedFees struct returned by calculate_fee. The fee should always be equal to the user_fee_rate passed in.
• Changed function signature of calculate_fee and so added None to other places was called.