Severe bug triggered by mplain with --presets-word

[readbeyond]

#Reported by QA Collective. Confirmed.

See: https://groups.google.com/forum/#!topic/aeneas-forced-alignment/NnpyiejGkHk

There are actually two different issues:

1. in the recursion for mplain it might happen that at level 2 a sentence with more than one word gets assigned a zero-length time interval: in that case, at level 3 a trivial tree should be returned, without running the DTW: this can be solved modifying executetask.py;

2. however, it might happen that a sentence gets assigned a non-zero-length interval (at level 2), but after MFCC masking, it becomes zero-length: this must be dealt with in dtw.py in the private _setup_dtw().

[readbeyond]

I have coded a tentative fix, I need to run tests before pushing to devel.

[readbeyond]

Fixed and merged on devel. Closing.