fix: add validation for find_free_time

[blaukc]

Added validation for start time < end time check for find_free_time

Validation can be added as setting a start time greater than end time will NOT result in no students being shown, which is an incorrect result (bug).

Can be allowed only if the current behavior causes the software to misbehave (i.e., crash, give incorrect results, store inconsistent data, or make it unusable for typical users).

Closes: #107, #79

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 54.54%. Comparing base (ce10f9d) to head (549784f).

Files	Patch %	Lines
...in/java/seedu/address/logic/parser/ParserUtil.java	0.00%	6 Missing ⚠️
...ddress/logic/parser/FindFreeTimeCommandParser.java	0.00%	2 Missing ⚠️
...a/seedu/address/model/student/IsFreePredicate.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #131      +/-   ##
============================================
- Coverage     54.76%   54.54%   -0.23%     
  Complexity      434      434              
============================================
  Files            98       98              
  Lines          1983     1991       +8     
  Branches        205      206       +1     
============================================
  Hits           1086     1086              
- Misses          861      869       +8     
  Partials         36       36              

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[taufiq]

Stellar work!