MAE-380: Monthly instalments

[erawat]

Overview

This PR adds:

• Calculator for fixed period monthly instalments.
• Calculator for fixed period annual instalments
• Logics to calculate number of instalment based on membership type configuration (for example, 1 instalment for rolling monthly /lifetime duration for rolling membership type, duration for fixed period membership type when schedule is monthly)
• Add additional validations to validate given membership types.
• Add unit tests
• Add php dockblock for existing functions.