Slightly too complicated activity diagram for adding student in DG

[fymbc]

I think the activity diagram is extremely comprehensive, which is good in some ways!

This is subjective but in my opinion,

1. Some steps can be coalesced. Accept and execute user input (first two steps) can be combined (maybe even with parse) to make it simpler. A more high-level activity diagram can be more palatable for readers. Likewise for creating student, creating AddCommand object, etc.

2. In relation to point 1 on high-level, I think we can abstract out the exact implementation and instead write it more generically (e.g. MESSAGE_USAGE, etc), to place less focus on variable names and more on its purpose.

3. [Minor] For invalid arg format maybe spell out to argument? :)