Add split_trains() method to more component classes

[takluyver]

This adds select_trains() to Timepix3 and XGM, and split_trains() to those plus DelayLineDetector and the pulse pattern components.

While running the tests on this, I discovered that generating the DLD mock data for tests was very slow, due to the way h5py does broadcasting, so the first commit is a workaround for that.

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.17%. Comparing base (327b117) to head (2d4b635).

Files with missing lines	Patch %	Lines
src/extra/components/pulses.py	66.66%	3 Missing ⚠️
src/extra/components/timepix.py	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
+ Coverage   73.90%   74.17%   +0.26%     
==========================================
  Files          35       35              
  Lines        6909     6969      +60     
==========================================
+ Hits         5106     5169      +63     
+ Misses       1803     1800       -3     

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[takluyver]

I've added basic tests, but it's not much more than a smoketest, I don't check accessing any data from the split parts.