Implement segmentation post-processing

[constantinpape]

For:

• removing objects with small z-extent ("extra pancakes")
• very small objects ( "lonely pixels/ lines that have not been erased correctly")
• removing holes in objects ("Missing Pixels within the cell volumes")

I can probably lift the post-processing functions from SynapseNet for this.

[constantinpape]

Optionally: filter border touching objects.