[Issue 130] Update the unix parse function to be specific to either seconds of milliseconds

[jama5262]

What does this PR do?

Resolved #130

Create two new functions and update the unix parse function to be specific to either seconds of milliseconds

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (a fix or feature that would cause existing functionality to change)

• Directed the PR to develop branch and not the master branch
• Wrote additional tests, if needed
• All tests have passed
• I have updated the documentation accordingly, if needed.

[codecov-commenter]

Codecov Report

Merging #135 (bc5842a) into develop (ea33349) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #135      +/-   ##
===========================================
- Coverage    93.72%   93.72%   -0.01%     
===========================================
  Files           33       33              
  Lines         1323     1322       -1     
===========================================
- Hits          1240     1239       -1     
  Misses          83       83              

Impacted Files	Coverage Δ
lib/src/jiffy.dart	91.09% <100.00%> (-0.03%)	⬇️

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