BufferWrapper: partitions w/ sig slicing

[sk1p]

Supersedes #1025.

Support partitions that don't contain whole signal elements / frames, but slice the signal dimension.

TODO

• Support in memory dataset
• Tests for included UDFs with sig-slicing partitions
• Maybe updates for other included UDFs?
• Update UDF documentation

Contributor Checklist:

• I have added or updated my entry in the creators.json file
• I have added a changelog entry for my contribution
• I have added/updated documentation for all user-facing changes
• I have added/updated test cases
• I have included the rebuilt production build of the client (only if changes were made to the GUI)

Reviewer Checklist:

• /azp run libertem.libertem-data passed

[codecov]

Codecov Report

Merging #1153 (8a5ea2a) into master (ea22b51) will decrease coverage by 7.52%.
The diff coverage is 91.58%.

@@            Coverage Diff             @@
##           master    #1153      +/-   ##
==========================================
- Coverage   70.35%   62.83%   -7.53%     
==========================================
  Files         273      273              
  Lines       13281    13310      +29     
  Branches     1890     1893       +3     
==========================================
- Hits         9344     8363     -981     
- Misses       3581     4636    +1055     
+ Partials      356      311      -45     

Impacted Files	Coverage Δ
src/libertem/common/math.py	84.61% <50.00%> (-15.39%)	⬇️
src/libertem/io/dataset/base/tiling.py	83.47% <71.42%> (-0.88%)	⬇️
src/libertem/executor/base.py	77.87% <83.33%> (-0.51%)	⬇️
src/libertem/common/buffers.py	84.36% <92.85%> (+0.31%)	⬆️
src/libertem/udf/base.py	93.99% <100.00%> (+0.01%)	⬆️
src/libertem/udf/masks.py	85.91% <100.00%> (+0.40%)	⬆️
src/libertem/udf/sumsigudf.py	76.92% <100.00%> (+4.19%)	⬆️
src/libertem/io/dataset/dm.py	32.11% <0.00%> (-55.48%)	⬇️
src/libertem/io/dataset/k2is.py	28.48% <0.00%> (-48.76%)	⬇️
src/libertem/io/dataset/mrc.py	43.69% <0.00%> (-46.22%)	⬇️
... and 23 more

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