Add list-group command to display all student groups

[teekoksiang]

• Add list-group command to display all student groups
• Add test cases for testing ListGroupCommand

[jieffyy]

Great work, almost LGTM 👍 Have some clarifications to make regarding the intended usage though.

[codecov-io]

Codecov Report

Merging #126 into master will increase coverage by 0.27%.
The diff coverage is 91.30%.

@@             Coverage Diff              @@
##             master     #126      +/-   ##
============================================
+ Coverage     68.72%   69.00%   +0.27%     
- Complexity      535      542       +7     
============================================
  Files            93       94       +1     
  Lines          1848     1871      +23     
  Branches        207      209       +2     
============================================
+ Hits           1270     1291      +21     
  Misses          490      490              
- Partials         88       90       +2     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)	1.00 <0.00> (ø)
...seedu/address/logic/commands/ListGroupCommand.java	90.47% <90.47%> (ø)	5.00 <5.00> (?)
.../seedu/address/logic/parser/AddressBookParser.java	80.95% <100.00%> (+0.95%)	13.00 <0.00> (+1.00)
...rc/main/java/seedu/address/model/ModelManager.java	86.66% <100.00%> (+0.22%)	26.00 <1.00> (+1.00)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5bb3d15...eddc1c4. Read the comment docs.

[jieffyy]

Nice, LGTM 👍