Reconstruct trigger time in traces

Add function to make arrival times relative to the timestamp.
Currently does nothing and is not used.
Implement function to reconstruct trigger time from traces
Put table description in ProcessEvents class
Move trigger reconstruction to new class and clean up

Remove use of np.nan, just assign -999 directly
Convert trace indexes to ns in _reconstruct_time_from_traces
Move trigger reconstruction to new class
Fix bugs in finding trigger time

Fix description: dict.update() returns None, but the dict is updated,
so don’t assign, just update.

Fix filling t_trigger column: getattr(table.cols, ‘’) gets the column,
but not the array of elements, so add [:].

Fix not finding trigger: first_above_threshold now returns -999,
when no trigger found, so use that instead of -1.

Disable debug messages
Add tests for ProcessEvents and ProcessEventsWithTriggerOffset

Test data currently not included because it is to big,
Run the script in the README.md to get the PE-testdata.h5
I’ll try to create a smaller data sample that can be included (less
traces?)
Fix ProcessEventsWithLINT

This should fix ProcessEventsWithLINT
when i == 0: x0 would get the last element, which could be anything
that made y0 >= y1 possible, possible divide by zero, etc..
this is now prevented.
Add tests for other ProcessEvents subclasses

Remove unused imports
The test ProcessIndexedEventsTests.test_process_traces works,
however, it causes progressbar output, redirecting stdout doesn’t work,
so for now it is disabled.
Fix progressbar output showing up in tests
Add code example to process_events

Add example code for process_events
Fix example for coincidences
Merge branch 'master' into trigger-timings
Skip ProcessEventsTests if no test datafile exists

The PE-testdata.h5 file is >50 MB.
A smaller sample should be created for this test.
Small enough that it can be added to the repository,
once that is done and the tests have been adapted,
it can be removed from the .gitignore to be included.

For now skip tests if the file is not available,
otherwise tests on Travis will take a lot longer.
Version bump to 0.9.6
Merge pull request #9 from HiSPARC/trigger-timings

Reconstruct trigger time in traces when processing events (new subclass).
Result is stored in new t_trigger column.
t_trigger is relative to the start of the trace, like the t# columns.
-999 if no trigger can be found, due to data filter or different thresholds.