Add dummy segmentation extractor

[h-mayorquin]

I am adding this as a general testing tool that will help us with the testing and refactoring of the segmentation interfaces in nwb conversion tools. The code is documented and has a simple test with it.

I also changed other small mistakes, errors in the code such as wrong documentation in the base class and inconsistent typing requirements in the NumpySegmentationExtractor

[codecov]

Codecov Report

Merging #176 (5282f3a) into master (4ba3c56) will increase coverage by 2.13%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
+ Coverage   71.04%   73.18%   +2.13%     
==========================================
  Files          33       33              
  Lines        2069     2088      +19     
==========================================
+ Hits         1470     1528      +58     
+ Misses        599      560      -39     

Flag	Coverage Δ
unittests	73.18% <95.65%> (+2.13%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/roiextractors/segmentationextractor.py	85.55% <ø> (ø)
src/roiextractors/testing.py	99.24% <95.23%> (-0.76%)	⬇️
...tors/extractors/numpyextractors/numpyextractors.py	60.47% <100.00%> (+23.95%)	⬆️