Update Network Traffic for CS03

[clenk]

• end must be greater than or equal to start
• extensions and end are now id-contributing properties

[codecov-commenter]

Codecov Report

Merging #514 (7a9d052) into master (646aaa3) will not change coverage.
The diff coverage is 33.33%.

@@           Coverage Diff           @@
##           master     #514   +/-   ##
=======================================
  Coverage   87.12%   87.12%           
=======================================
  Files         151      151           
  Lines       17702    17702           
=======================================
  Hits        15423    15423           
  Misses       2279     2279           

Impacted Files	Coverage Δ
stix2/v21/observables.py	34.51% <33.33%> (ø)

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