[v1.3.2] Add support for Opening Document

[CodyChew]

• OpenCommand only has Case and Doc types with one argument, so there is only one parser to return the appropriate commands
• OpenDocumentCommand checks for valid index and tries to open the doc
• CommandExceptions thrown:
  • invalid document index
  • reference does not exist
  • error opening file
  • Desktop API not supported

[codecov-io]

Codecov Report

Merging #155 into master will decrease coverage by 0.93%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #155      +/-   ##
============================================
- Coverage     55.29%   54.36%   -0.94%     
- Complexity      501      502       +1     
============================================
  Files           115      116       +1     
  Lines          2217     2255      +38     
  Branches        299      304       +5     
============================================
  Hits           1226     1226              
- Misses          858      895      +37     
- Partials        133      134       +1     

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/seedu/pivot/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...c/main/java/seedu/pivot/commons/util/FileUtil.java	59.37% <0.00%> (-16.63%)	11.00 <0.00> (+1.00)	⬇️
.../main/java/seedu/pivot/logic/commands/Command.java	100.00% <ø> (ø)	1.00 <0.00> (ø)
...commands/documentcommands/OpenDocumentCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...va/seedu/pivot/logic/parser/OpenCommandParser.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...seedu/pivot/model/investigationcase/Reference.java	56.25% <0.00%> (-3.75%)	5.00 <0.00> (ø)
...ain/java/seedu/pivot/storage/ReferenceStorage.java	71.42% <ø> (ø)	2.00 <0.00> (ø)

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[Vielheim]

LGTM