Enable optional processing of sequences with <4 heart beats.

[id-b3]

Describe the solution you'd like
In NK2 versions <=0.2.4 short ECG sequences were segmented in the ecg_segment.py file.
In versions 0.2.5 and up, this no longer works due to the addition of these lines.

How could we do it?
Make it optional to allow processing of short ECG sequences.

[welcome]

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