Sherwin refactor sort policy comparators, commission for download and command messages

[sherrpass]

• Enhanced sortpolicy comparators for to calculate total payment and total commission.
• Refactored commission calculation for download commnad
• Improve command messages for several commands

[yatbun]

To standardise output messages. I will look at how to show empty lists better.

[codecov-commenter]

Codecov Report

Merging #150 (14f65c0) into master (8c4035d) will decrease coverage by 0.90%.
The diff coverage is 11.76%.

@@             Coverage Diff              @@
##             master     #150      +/-   ##
============================================
- Coverage     50.55%   49.64%   -0.91%     
+ Complexity      506      503       -3     
============================================
  Files           106      107       +1     
  Lines          2243     2270      +27     
  Branches        273      280       +7     
============================================
- Hits           1134     1127       -7     
- Misses         1050     1083      +33     
- Partials         59       60       +1     

Impacted Files	Coverage Δ
src/main/java/seedu/siasa/MainApp.java	0.00% <0.00%> (ø)
...c/main/java/seedu/siasa/commons/core/Messages.java	0.00% <ø> (ø)
...in/java/seedu/siasa/commons/util/CurrencyUtil.java	0.00% <0.00%> (ø)
.../java/seedu/siasa/logic/commands/ClearCommand.java	14.28% <0.00%> (-85.72%)	⬇️
...va/seedu/siasa/logic/commands/DownloadCommand.java	0.00% <0.00%> (ø)
...n/java/seedu/siasa/logic/commands/ExitCommand.java	100.00% <ø> (ø)
...iasa/logic/commands/contact/AddContactCommand.java	66.66% <ø> (ø)
...a/logic/commands/contact/DeleteContactCommand.java	100.00% <ø> (ø)
...asa/logic/commands/contact/EditContactCommand.java	97.01% <ø> (ø)
...gic/commands/contact/ListContactPolicyCommand.java	0.00% <0.00%> (ø)
... and 28 more

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[sherrpass]

LGTM for your changes yatbun, but I think we definitely need to boost our test cases in a future PR.

[zhuoyang125]

LGTM!