Update gem & science commands to support auto update

[mhdsyfq77]

Several changes made but main resolves #159

Main change:

• Gem & science lists now updates whenever a module is added
• Moved GemCommandStorage methods to GemCommand

Other changes:

• Added module title to display (i feel like users would like to see module title if not they just see letters and numbers x.x)
• Shifted gem command messages to commons.core.Messages
• Separated display of GE commands using the GE pillars (Human Cultures, Asking Questions, etc.) as gem module codes look too congested
• Resolved test issues that came with changes

[codecov-io]

Codecov Report

Merging #182 (c598282) into master (3a7461e) will decrease coverage by 0.03%. The diff coverage is 87.87%.

@@             Coverage Diff              @@
##             master     #182      +/-   ##
============================================
- Coverage     75.67%   75.64%   -0.04%     
  Complexity      586      586              
============================================
  Files            96       96              
  Lines          1768     1774       +6     
  Branches        183      185       +2     
============================================
+ Hits           1338     1342       +4     
  Misses          345      345              
- Partials         85       87       +2     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...java/seedu/address/logic/parser/GradPadParser.java	69.23% <ø> (ø)	14.00 <0.00> (ø)
...in/java/seedu/address/storage/GemCommandPaths.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
.../java/seedu/address/storage/GemCommandStorage.java	100.00% <ø> (+4.25%)	14.00 <0.00> (-5.00)	:arrow_up:
...a/seedu/address/logic/commands/ScienceCommand.java	83.33% <80.00%> (-4.17%)	5.00 <0.00> (ø)
.../java/seedu/address/logic/commands/GemCommand.java	89.13% <87.50%> (-3.98%)	11.00 <5.00> (+5.00)	:arrow_down:
.../seedu/address/logic/commands/RequiredCommand.java	97.39% <100.00%> (+0.04%)	36.00 <0.00> (ø)

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