-Edited add function to take in start date and end date
edit date parser function
Changes to parser and commands
- DateParser will depend on Natty  and format of date is being force to a certain format yyyy/MM/dd
- edit Edit function and add function for the new date parser
Revamped ToDo model.
Added TestCases for ToDo model.
Added PrettyTimeParserWrapper to reduce delay in Natty processing.
Other misc. function for DateParser.
Revamped DateParser implementation.
- Changed `ToDoAdd` to use new DateParser implementation.
- Added `InvalidDateException` will be thrown if date is not valid.
Fixes ToDo model test case.
Cherry picked test cases from New_Edit_Function.
Fixes regression on `ToDoEdit` and `EditFunctionTest`
Fixes regression to `AddFunctionTest` due to LocalDate to LocalDateTime changes.
Fixes #59. Where LocalDateTime isn't serializing and deserializing properly.
Change made in these files
- edit Edit Function to take in task type, is able to handle null problem and also is able to remove start and end date
- implement the removeStartDate and removeEndDate method
- change of Edit function test cases
Updated ToDo model.
- Forced `INVALID_DATE` to max.
- Replaced `id` int with UUID.
Updated Logic with TreeMap implementation to display todo sorted by date.
Fixes regression due to `todoMapDisplay` being null.
Changes in Edit Function
- minor changes to the logic problem
- change magic numbers to enum constants
- test cases for null inputs
- test cases for new enum constants
Revert "Changes in Edit Function"

This reverts commit 1a13fce58ab518af11369f81484b77d94a0b116f.
Changes in Edit Function
- minor changes to the logic problem
- change magic numbers to enum constants
- test cases for null inputs
- test cases for new enum constants
Amend edit function