Separate command validation from execution

[ryoarmanda]

Summary:

• The Command class now has two abstract methods, validate and execute, and one concrete method, run.
• run executes the command with validation (just like the previous execute) and execute is the same but with NO validation.
• validate and execute are supposed to be internal methods, while run is exposed for public use (e.g. LogicManager, tests, etc).

Reason for separation:

• Separates validation from business logic
• Helps with undo/redo functionality in Implement undo/redo functionalities with model history #74 , so I can override run to store the model's state after validation but before execution.

I have already modified test names and utilities to adhere to this naming. All tests have passed.

Implications for future developments:

• All commands have to implement validate even when it does not need validation (look at what I did for exit, etc).
• You still develop new commands the same way as usual, with execute.

[coveralls]

Pull Request Test Coverage Report for Build 89

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 14 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.3%) to 60.409%

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Files with Coverage Reduction	New Missed Lines	%
file:/home/travis/build/AY1920S1-CS2103T-T11-1/main/src/main/java/seedu/address/logic/commands/EditCommand.java	1	98.33%
file:/home/travis/build/AY1920S1-CS2103T-T11-1/main/src/main/java/seedu/address/logic/commands/RemindersCommand.java	3	0.0%
file:/home/travis/build/AY1920S1-CS2103T-T11-1/main/src/main/java/seedu/address/logic/commands/BudgetCommand.java	5	0.0%
file:/home/travis/build/AY1920S1-CS2103T-T11-1/main/src/main/java/seedu/address/logic/commands/EventCommand.java	5	0.0%

Totals
Change from base Build 74:	0.3%
Covered Lines:	1033
Relevant Lines:	1710

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💛 - Coveralls

[qweiping31415]

Noted on the changes to split old execute command(currently called run), into a method validate that throws Exception if the command is not executable, and execute only when it passes validate.