Modify implementation of addscreening

[coveralls]

Pull Request Test Coverage Report for Build 128

• 0 of 43 (0.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.09%) to 76.965%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/seedu/address/model/ModelManager.java	0	1	0.0%
src/main/java/seedu/address/logic/parser/ParserUtil.java	0	2	0.0%
src/main/java/seedu/address/model/screening/Screening.java	0	3	0.0%
src/main/java/seedu/address/model/MoviePlanner.java	0	5	0.0%
src/main/java/seedu/address/logic/commands/AddScreeningCommand.java	0	32	0.0%
<!--	Total:	0	43	0.0%	-->

Totals
Change from base Build 126:	0.09%
Covered Lines:	1978
Relevant Lines:	2570

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💛 - Coveralls