Vector-like regionprops attributes saved as multiple columns.

[JoOkuma]

This issues comes from #259 (comment)

I feel sometimes we'd like to filter the nodes by the intensities of specific channels. Would it make sense to flatten the vector-like props? regionprops_table automatically does this, and I was wondering if we could use this instead.

This could be very useful for filtering, where unpickling or comparing arrays is more complicated.

IMO, this could be optional, because high-dimensional vectors are better stored as unit many columns could be detrimental.