support different time formats in Annotations.to_data_frame()

[drammock]

it's working, but a bit hackish, so marking as draft while I clean it up.
closes #12274

In [1]: import mne
In [2]: foo = mne.Annotations(onset=[1,5,11], duration=[2, 2, 3], description=list('abc'))
In [3]: foo.to_data_frame(None)
Out[3]: 
   onset  duration description
0    1.0       2.0           a
1    5.0       2.0           b
2   11.0       3.0           c
In [4]: foo.to_data_frame("datetime")
Out[4]: 
                onset  duration description
0 1970-01-01 00:00:01       2.0           a
1 1970-01-01 00:00:05       2.0           b
2 1970-01-01 00:00:11       3.0           c
In [5]: foo.to_data_frame("ms")
Out[5]: 
   onset  duration description
0   1000       2.0           a
1   5000       2.0           b
2  11000       3.0           c
In [6]: foo.to_data_frame("timedelta")
Out[6]: 
            onset  duration description
0 0 days 00:00:01       2.0           a
1 0 days 00:00:05       2.0           b
2 0 days 00:00:11       3.0           c

[drammock]

@dengemann does this meet your needs?

[dengemann]

Looks awesome! This will facilitate interactive prototyping of analyses of signal windows with irregular duration. LGTM!

[agramfort]

❤️

[larsoner]

Nice, I think it just needs a devel.rst update, feel free to mark for merge-when-green once you push one @drammock !