Add an automatic detection of the number of stream

[vmarandon]

The detection of the number of stream is done by opening a file, then get the median of difference between event ids.
I use multiple event in case there is an issue or a missing event.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.43%. Comparing base (cb5646d) to head (31f990f).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ctapipe_io_nectarcam/__init__.py	77.77%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
- Coverage   85.60%   85.43%   -0.17%     
==========================================
  Files           7        7              
  Lines         917      934      +17     
==========================================
+ Hits          785      798      +13     
- Misses        132      136       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.