Update equality check for person

[Shruthiiii03]

2 students are now considered the same person only if they are in the same class and have the same register number.
Fixes #258

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ	Complexity Δ
...c/main/java/seedu/address/model/person/Person.java	98.76% <100.00%> (+0.04%)	42.00 <6.00> (+2.00)

... and 1 file with indirect coverage changes

[gohqingkhang]

LGTM, but someone else to check also, as need to be more careful about codebase change