fix slope calculation in multilinear interpol

[PSeitz]

fix slope calculation in multilinear interpol
add test to check for compression

[codecov]

Codecov Report

Merging #1161 (fa5b062) into main (46b86a7) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1161      +/-   ##
==========================================
+ Coverage   93.77%   93.81%   +0.03%     
==========================================
  Files         203      203              
  Lines       33654    33899     +245     
==========================================
+ Hits        31560    31801     +241     
- Misses       2094     2098       +4     

Impacted Files	Coverage Δ
fastfield_codecs/src/lib.rs	96.87% <100.00%> (ø)
fastfield_codecs/src/linearinterpol.rs	98.63% <100.00%> (+0.05%)	⬆️
fastfield_codecs/src/multilinearinterpol.rs	98.02% <100.00%> (-0.27%)	⬇️
src/indexer/index_writer.rs	96.98% <0.00%> (-0.26%)	⬇️
common/src/bitset.rs	98.46% <0.00%> (-0.16%)	⬇️
src/indexer/merger.rs	98.75% <0.00%> (ø)
src/positions/serializer.rs	100.00% <0.00%> (ø)
src/indexer/segment_entry.rs	90.00% <0.00%> (ø)
src/fastfield/delete.rs
... and 6 more

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