Add A-Assertions

Parser performCommand: Code quality has not been achieved because method is too long and OOP is not fully achieved 

PerformCommand is supposed to handle the general processing of command, checking the performing comman shall be done in separate classes for each different cases. 

Created several classes in Parser to handle different types of commands to make the code quality better, ensuring methods are not too long and as well as to make full use of OOP.

…omised path for my hard disk

The user using Duke might not have same file path in local hard disk

The file path has thus been changed to ./data/duke.txt which makes it more general than the previous iteration

Let's create a new text file in ./data/duke.txt instead of C:\Users\linwe\Documents\taskSaved.txt!

…of the validity of inputs are not robust

Checking not being robust will result in Duke mis-intepereting an input as valid when it is not, thereby causes execption being thrown without catching

Included more robust checking to ensure the inputs are in correct format, before a task object is created

Let's check the inputs by trying throwing and catching exception when inputs are incorrect!

Same tasks can be added multiple times

Add C-DetectDuplicates

It is more user friendly so that users would not accidentally add
same tasks into the task list.

Let's
* Check whether there are existing same task in task list as the task
that user has keyed in
* Include the equals function in task to check whether two tasks are
the same or not

the interaction with User and not the parser

and improved implementation of the different tasks

Improve code style and quality and passed checkstyle

Entering dates before the current date is not allowed
For event, start date cannot be after end date

Settle case when input is missing or is filled with spaces