Add missing header comments

[RobotWizzard]

Add missing header comments in:

• EditRentalCommand
• FindCommand
• AddressBook
• Client

Fix checkstyle issues.

Fixes #110 Fixes #112

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 51.89873% with 38 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...u/address/logic/parser/AddRentalCommandParser.java	0.00%	22 Missing :warning:
...u/address/logic/parser/AddClientCommandParser.java	62.96%	8 Missing and 2 partials :warning:
...eedu/address/logic/commands/EditClientCommand.java	0.00%	3 Missing :warning:
.../address/logic/parser/EditClientCommandParser.java	92.00%	0 Missing and 2 partials :warning:
...eedu/address/logic/commands/EditRentalCommand.java	0.00%	1 Missing :warning:

Files with missing lines	Coverage Δ	Complexity Δ
...in/java/seedu/address/commons/util/RentalUtil.java	94.44% <ø> (ø)	4.00 <0.00> (ø)
...seedu/address/logic/commands/AddRentalCommand.java	0.00% <ø> (ø)	0.00 <0.00> (?)
...java/seedu/address/logic/commands/FindCommand.java	38.88% <ø> (-61.12%)	2.00 <0.00> (-4.00)
.../address/logic/parser/EditRentalCommandParser.java	0.00% <ø> (ø)	0.00 <0.00> (?)
...in/java/seedu/address/logic/parser/ParserUtil.java	58.73% <ø> (-38.50%)	16.00 <0.00> (+2.00)	:arrow_down:
src/main/java/seedu/address/model/AddressBook.java	71.42% <ø> (-11.91%)	11.00 <0.00> (ø)
...c/main/java/seedu/address/model/client/Client.java	93.47% <ø> (ø)	19.00 <0.00> (?)
...rc/main/java/seedu/address/model/client/Email.java	90.00% <ø> (ø)	11.00 <0.00> (?)
.../java/seedu/address/storage/JsonAdaptedPerson.java	83.92% <100.00%> (-16.08%)	10.00 <0.00> (-1.00)
...n/java/seedu/address/ui/RentalInformationCard.java	0.00% <ø> (ø)	0.00 <0.00> (?)
... and 5 more

... and 3 files with indirect coverage changes

[RobotWizzard]

codecov is expected to fail due to missing JUnit tests.