Improve test coverage

[yan-soon]

Improved the Test Coverage for the Commons and Logic Package.

Total test coverage is up from 75% to 79%.

Notable changes for Commons Package

• the addition of the GuiSettingsTest and LogsCenterTest classes.
• added tests for Config, GuiSettings, LogsCenter, and Version classes.

Notable changes for the Logic Package

• addition of YesCommandTest classes.
• added tests for ClearCommand , CommandResult and YesCommand classes.
• added tests for DeleteCommandParser, SearchCommandParser and GradPadParser classes.
• added HandleStall() and assignStalledComponents() methods in Logic interface.
• added tests for LogicManager class

[codecov-io]

Codecov Report

Merging #197 (7ec176b) into master (48d80b1) will increase coverage by 3.50%. The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master     #197      +/-   ##
============================================
+ Coverage     75.87%   79.38%   +3.50%     
- Complexity      587      628      +41     
============================================
  Files            96       96              
  Lines          1774     1775       +1     
  Branches        185      185              
============================================
+ Hits           1346     1409      +63     
+ Misses          341      290      -51     
+ Partials         87       76      -11     

Impacted Files	Coverage Δ	Complexity Δ
...n/java/seedu/address/commons/core/GuiSettings.java	79.31% <ø> (+17.24%)	11.00 <0.00> (+5.00)
...rc/main/java/seedu/address/logic/LogicManager.java	96.07% <0.00%> (+56.86%)	20.00 <0.00> (+15.00)
.../java/seedu/address/logic/commands/GemCommand.java	89.13% <ø> (ø)	11.00 <0.00> (ø)
.../seedu/address/logic/commands/RequiredCommand.java	97.39% <ø> (ø)	36.00 <0.00> (ø)
...a/seedu/address/logic/commands/ScienceCommand.java	83.33% <ø> (ø)	5.00 <0.00> (ø)
.../java/seedu/address/storage/GemCommandStorage.java	100.00% <ø> (ø)	14.00 <0.00> (ø)
.../seedu/address/storage/RequiredCommandStorage.java	100.00% <ø> (ø)	16.00 <0.00> (ø)
...in/java/seedu/address/commons/core/LogsCenter.java	89.74% <100.00%> (+13.42%)	13.00 <1.00> (+3.00)
...va/seedu/address/logic/commands/CommandResult.java	100.00% <0.00%> (+6.66%)	10.00% <0.00%> (+1.00%)
... and 7 more

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