Fix DG features sequence diagrams

[jiahui0309]

Modified the sequence diagrams, added logging statements. Closes #224.

This PR does not include (as of latest commit):

• any changes to activity diagrams

This PR includes:

• logging
• DG sequence diagrams
• changes to DG implementation content for add, addnote, find

[drustanyjt]

@jiahui0309 why do the XYZCommandParser have crosses for object deletion but some of the other things (like the XYZCommand objects) do not? Should we add object deletions for the XYZCommand too after r is returned?

[jiahui0309]

@drustanyjt okay! initially i was following the diagram they provided, but it seems reasonable to delete the obj after returning the result

[tengcharmaine]

LGTM