Rename polyphase_matrix() to polyphase_decompose()

[mhostetter]

Closes #152

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (19185b9) 77.50% compared to head (dec4a65) 77.56%.

Files	Patch %	Lines
src/sdr/_filter/_design_multirate.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##           release/0.0.x     #159      +/-   ##
=================================================
+ Coverage          77.50%   77.56%   +0.06%     
=================================================
  Files                 42       42              
  Lines               2058     2055       -3     
=================================================
- Hits                1595     1594       -1     
+ Misses               463      461       -2     

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