Delete cached index if attrs change, add a get exclusive end method

[jkanche]

resolves #60

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.04%. Comparing base (c94eee5) to head (e05cbc4).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/iranges/IRanges.py	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
+ Coverage   66.55%   67.04%   +0.49%     
==========================================
  Files           4        4              
  Lines         867      877      +10     
  Branches      164      166       +2     
==========================================
+ Hits          577      588      +11     
- Misses        212      213       +1     
+ Partials       78       76       -2     

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