379 add avail command

[Echomo-Xinyu]

• Implements the avail command
• Include test case for the relevant logic
• minor reorganization of code base

[codecov]

Codecov Report

Base: 76.23% // Head: 76.07% // Decreases project coverage by -0.15% ⚠️

Coverage data is based on head (aba93a6) compared to base (a76d905).
Patch coverage: 75.48% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #381      +/-   ##
============================================
- Coverage     76.23%   76.07%   -0.16%     
- Complexity      741      789      +48     
============================================
  Files            94       97       +3     
  Lines          2268     2408     +140     
  Branches        279      330      +51     
============================================
+ Hits           1729     1832     +103     
- Misses          445      453       +8     
- Partials         94      123      +29     

Impacted Files	Coverage Δ
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)
...rc/main/java/seedu/address/model/ModelManager.java	94.36% <0.00%> (-2.74%)	⬇️
src/main/java/seedu/address/model/TeachersPet.java	88.88% <0.00%> (-2.54%)	⬇️
...c/main/java/seedu/address/model/student/Class.java	86.51% <ø> (-0.99%)	⬇️
...ava/seedu/address/logic/commands/AvailCommand.java	44.44% <44.44%> (ø)
.../java/seedu/address/model/timerange/TimeRange.java	69.23% <69.23%> (ø)
...seedu/address/model/student/UniqueStudentList.java	82.39% <78.02%> (-9.77%)	⬇️
.../seedu/address/logic/parser/TeachersPetParser.java	86.36% <83.33%> (-4.12%)	⬇️
...in/java/seedu/address/logic/parser/ParserUtil.java	92.80% <92.30%> (+1.33%)	⬆️
...seedu/address/logic/parser/AvailCommandParser.java	100.00% <100.00%> (ø)
... and 1 more

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