Warnings during feature extraction about sleepecg HOT 3 CLOSED

Avoiding the warnings turned out to be very cumbersome, so I made the changes to ignore them in #142.

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Those warnings occur for analysis windows where there are no heartbeats (or just heartbeats with equal distances, voiding some statistical measures), in which case the resulting feature value is nan. We should be able to avoid them by removing those windows from NN before calculating the features (and just directly set the respective entries in the feature matrix to np.nan). Features with value nan are masked in prepare_data_keras, so this does not influence the classification.

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Alternatively, we could also catch (and silence) those warnings if there is nothing to warn about. Which solution would you prefer?

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