Delete concert command

[wes-w-z-h]

• [x] new command to delete concerts
• [x] remove from storage
• [x] remove from concert contacts

Fixes #68

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Files with missing lines	Coverage Δ	Complexity Δ
...u/address/logic/commands/DeleteConcertCommand.java	100.00% <100.00%> (ø)	7.00 <7.00> (?)
...du/address/logic/commands/DeletePersonCommand.java	100.00% <100.00%> (ø)	7.00 <1.00> (?)
.../seedu/address/logic/parser/AddressBookParser.java	100.00% <100.00%> (ø)	15.00 <0.00> (+1.00)
...dress/logic/parser/DeleteConcertCommandParser.java	100.00% <100.00%> (ø)	2.00 <2.00> (?)
...ddress/logic/parser/DeletePersonCommandParser.java	100.00% <100.00%> (ø)	2.00 <1.00> (?)
src/main/java/seedu/address/model/AddressBook.java	67.74% <100.00%> (+4.40%)	20.00 <1.00> (+2.00)
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...rc/main/java/seedu/address/model/ModelManager.java	77.50% <100.00%> (+3.14%)	29.00 <1.00> (+2.00)

... and 1 file with indirect coverage changes