Change implementation of email checking

[ktaekwon000]

Fixes #215

[codecov]

Codecov Report

Merging #250 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #250   +/-   ##
=========================================
  Coverage     61.87%   61.87%           
  Complexity      770      770           
=========================================
  Files           120      120           
  Lines          3019     3019           
  Branches        350      350           
=========================================
  Hits           1868     1868           
  Misses         1032     1032           
  Partials        119      119           

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/seedu/address/model/patient/Email.java	86.66% <100.00%> (ø)	8.00 <0.00> (ø)

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[gsmoon97]

LGTM! nice job