add check for trajectory changes

[janash]

this commit adds a decorator which can be used to check if the simulation file has been changed by the test that is being run.

[codecov-commenter]

Codecov Report

Merging #40 (c53ca62) into master (87d4477) will not change coverage.
The diff coverage is n/a.

❗ Current head c53ca62 differs from pull request most recent head fa34dfa. Consider uploading reports for the commit fa34dfa to get more accurate results

@@           Coverage Diff           @@
##           master      #40   +/-   ##
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  Coverage   95.57%   95.57%           
=======================================
  Files           2        2           
  Lines         226      226           
=======================================
  Hits          216      216           
  Misses         10       10           

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