Take info on prescaler and trigger from previous subrun if not available in the current file

[aleberti]

Closes #85 .

[codecov]

Codecov Report

Attention: 35 lines in your changes are missing coverage. Please review.

Comparison is base (bff758a) 89.57% compared to head (dfd7fdb) 88.32%.
Report is 10 commits behind head on master.

❗ Current head dfd7fdb differs from pull request most recent head f336e9f. Consider uploading reports for the commit f336e9f to get more accurate results

Files	Patch %	Lines
ctapipe_io_magic/__init__.py	61.11%	35 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
- Coverage   89.57%   88.32%   -1.25%     
==========================================
  Files          10       10              
  Lines        1103     1165      +62     
==========================================
+ Hits          988     1029      +41     
- Misses        115      136      +21     

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[jsitarek]

fine as it is, but I left two comments for possible improvements.