Fix Delete Client Indexing

[Syasyazman]

Also re-added test cases for delete client and order.

[codecov-io]

Codecov Report

Merging #79 into master will increase coverage by 3.91%.
The diff coverage is 63.63%.

@@             Coverage Diff              @@
##             master      #79      +/-   ##
============================================
+ Coverage     53.84%   57.76%   +3.91%     
- Complexity      399      428      +29     
============================================
  Files            90       91       +1     
  Lines          1679     1688       +9     
  Branches        173      174       +1     
============================================
+ Hits            904      975      +71     
+ Misses          727      661      -66     
- Partials         48       52       +4     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/seedu/address/model/AddressBook.java	74.60% <33.33%> (+23.78%)	21.00 <0.00> (+5.00)
...model/person/OrderIdContainsKeywordsPredicate.java	71.42% <71.42%> (ø)	2.00 <2.00> (?)
...rc/main/java/seedu/address/model/person/Order.java	46.66% <100.00%> (+46.66%)	4.00 <0.00> (+4.00)
...in/java/seedu/address/logic/parser/ParserUtil.java	80.00% <0.00%> (+4.00%)	16.00% <0.00%> (+1.00%)
...rc/main/java/seedu/address/model/ModelManager.java	85.71% <0.00%> (+7.93%)	26.00% <0.00%> (+3.00%)
...va/seedu/address/model/person/UniqueOrderList.java	43.90% <0.00%> (+21.95%)	10.00% <0.00%> (+5.00%)
...du/address/logic/commands/DeleteClientCommand.java	100.00% <0.00%> (+30.76%)	8.00% <0.00%> (+1.00%)
... and 1 more

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