Payment calculation module

[LeHack]

We need a module that will be able to handle the monthly fee calculations for a attendee using the following rules:

• to calculate the monthly fee, first check what type of payment the attendee has, if it's monthly then it's a fixed fee and nothing else needs to be calculated for the given month
• if it's attendance-based, then we need to know how many times the attendee was present during that particular month and then we take the group monthly fee subtract the attendance count x10zł, note that this number may come out negative and that's valid (then we get a surplus which advances for the next month)

Finally the module should be able to give us two numbers:

• the fee for any selected month (using the above rules)
• the total current balance - to calculate this we take every month for which there is any attendance info available for that particular attendee, multiply the month count by the group fee and subtract the attendance count x10zł (this should be always calculated live, not stored anywhere)