Fix large quantity bugs

[hakujitsu]

Closes #211

Changes Made

• Add additional validation for Unit Price and Quantity to ensure a max value of 10 million
• Amend existing test cases for Unit Price and Quantity
• Add test cases for parsing Sale related fields in parserUtil
• Refactor parsing methods for Unit Price and Quantity
• Fix documentation to specify the maximum value of Unit Price and Quantity
• Add dollar sign to UnitPrice toString method

[codecov-io]

Codecov Report

Merging #215 into master will increase coverage by 0.06%.
The diff coverage is 91.17%.

@@             Coverage Diff              @@
##             master     #215      +/-   ##
============================================
+ Coverage     71.50%   71.57%   +0.06%     
- Complexity     1319     1324       +5     
============================================
  Files           181      181              
  Lines          4422     4425       +3     
  Branches        624      623       -1     
============================================
+ Hits           3162     3167       +5     
- Misses         1041     1043       +2     
+ Partials        219      215       -4     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/seedu/address/model/sale/Sale.java	86.88% <0.00%> (ø)	26.00 <0.00> (ø)
.../main/java/seedu/address/model/sale/UnitPrice.java	81.81% <80.00%> (+6.81%)	16.00 <8.00> (+3.00)
...in/java/seedu/address/logic/parser/ParserUtil.java	97.47% <100.00%> (+0.08%)	40.00 <0.00> (ø)
...c/main/java/seedu/address/model/sale/Quantity.java	80.00% <100.00%> (+2.22%)	8.00 <4.00> (+2.00)
.../java/seedu/address/model/util/SampleDataUtil.java	37.50% <100.00%> (ø)	2.00 <0.00> (ø)
...in/java/seedu/address/storage/JsonAdaptedSale.java	98.14% <100.00%> (ø)	12.00 <0.00> (ø)

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[Asthenosphere]

Tested and works well, good to merge!