Non-sticky option for GreedyScheduler (#161)

[projekter]

This addresses #161, allowing an option to opt out of the task migration feature for the greedy scheduler. While I also mentioned the dynamic scheduler there, this probably makes a lot less sense - only if we could tell Julia not to migrate, but also only fix the thread id only once there are some resources left and not before the task starts.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 82.60870% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.14%. Comparing base (cba9127) to head (61606b6).
⚠️ Report is 150 commits behind head on master.

Files with missing lines	Patch %	Lines
src/implementation.jl	87.50%	2 Missing ⚠️
src/schedulers.jl	71.42%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #163      +/-   ##
==========================================
- Coverage   90.24%   89.14%   -1.10%     
==========================================
  Files           3        8       +5     
  Lines          82      645     +563     
==========================================
+ Hits           74      575     +501     
- Misses          8       70      +62     

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