Enable to skip seeds in region growing that are not concluded at end of time series

[DCHulskemper]

In the original method seeds that are not finished are not used for region growing. We should make it possible for online settings, but they should be skipped as a default setting.

suggestion:
Add skip_unfinished parameter, if True (default) don't add a seeds whose target index would be the last index of time series

[kathapand]

Relevant code part to consider skip_unfinished: https://github.com/3dgeo-heidelberg/py4dgeo/blob/debug_4dobc/src/py4dgeo/segmentation.py#L955