. Add a RemarkCommand that just throws an Exception.
. Add RemarkCommandTest that will test that executeUndoableCommand()
throws an Exception.
. Modify AddressBookParser to accept a RemarkCommand.
. Add new test to AddressBookParserTest, which will test that typing
"remark" returns an instance of RemarkCommand.

. Modify RemarkCommand to take in an Index and String for remark, and
print those two parameters as the error message
. Modify RemarkCommandTest to test the equals method
. Add RemarkCommandParser that will know how to parse two arguments, one
index and one with prefix 'r/'
. Add RemarkCommandParserTest that will test different boundary values
for RemarkCommandParser.
. Modify AddressBookParser to use the newly implemented
RemarkCommandParser
. Modify AddressBookParserTest to ensure that what the user input
generated the correct command.

. Add label with any random text inside PersonListCard.fxml
. Add FXML annotation in PersonCard to tie the variable to the actual
label.
. Modify PersonCardHandle so that future tests can read the remarks in
the cards.

. Add Remark to model component (copy from Address, remove the regex and
rename accordingly).
. Add test for Remark (to test the equals method).
. Modify RemarkCommand to now take in a Remark instead of a String.

. Add `Person#getRemark()`
. You may assume that the user will not be able to use the `add`
and `edit` commands to modify the remarks field (i.e. the person will
be created without a remark).
. Modify `SampleDataUtil` to add remarks for the sample data.

I recommend deleting your addressBook.xml after this step so that you
can see the remarks field come to life in your application.

. Add a new method `withRemark()` for `PersonBuilder`. This method will
create a new `Remark` for the person that it is currently building.
. Try and use the method on any sample `Person` in `TypicalPersons`.

. Just modify PersonCard's constructor to add the new method.
. For test, modify GuiTestAssert#assertCardDisplaysPerson(...) to
compare the now-functioning remark label.

. Replace the error message with the actual logic to modify the remarks
of a person.
. Update `RemarkCommandTest` to test that the `execute()` logic works.