teach prepare_intervals to output aclew-like intervals

[kalenkovich]

Right now, the way the function works emulates the way LENA creates intervals for lena5min.csv's. First and last intervals start and end outside of the recordings: 15:42:15:52 -> (15:40-15:45, 15:45-15:50, 15:50-15:55). They get trimmed afterward but they are still there while for ACLEW we don't want those short intervals at all. Keeping those intervals should be an option. Also, for ACLEW we'll need to introduce buffers to accommodate context regions.

• add an option to drop start and end intervals if they are shorter than required,
• add an option to introduce buffers before and after all regions.

See vihi-sampling code where intervals are created for the seedlings corpus.