fix(bug): Fix number format exception for `modify`, `delete`, `take`, `untake`

[wenenhoe]

Checklist for feature implementation and modification

modify: Fixes #154, fixes #155, fixes #179 delete: Fixes #194, fixes #204 take: Fixes #156, fixes #195

• [x] Added relevant milestone (under milestone tab on the right)
• Implementation
  • [ ] Sources (if any) credited under both relevant section(s) and Developer Guide (Acknowledgement)
  • [x] JUnit Test(s)
• Documentation
  • [x] JavaDoc for classes and methods
  • Developer Guide
  • [ ] Design (Can add as a todo statement for later on)
  • [ ] Implementation (Can add as a todo statement for later on)
  • [ ] Relevant User Story present
  • [ ] Non-functional requirement(s) that the feature addresses (if any)
  • [ ] Glossary of the (technical) term(s) used (if any)
  • [ ] Specialised instructions for manual testing (if any)
  • User Guide
  • [ ] Description and usage of feature (if any)
  • Contributions (under /docs/team/*.md)
  • [x] Code/Feature/Enhancement contribution
  • [ ] Developer Guide contribution
  • [ ] User Guide contribution
  • [x] Misc and team-based admin tasks (if any)

Additional Information

• Refactor obtaining list index and period to static methods in Command class