Added function and fixed bug in PerfectCube.java

[TaranjeetSinghKalsi]

Added another function to PerfectCube.java and fixed a testing mistake in line number 9

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[TaranjeetSinghKalsi]

The code looks good, could you please add some JUnit tests? (see https://github.com/TheAlgorithms/Java/tree/master/src/test/java/com/thealgorithms)

Thanks, I have added JUnit tests and also fixed a bug in isPerfectCube() function for negative numbers. Kindly look after it.

[siriak]

@debasishbsws please review