Rename BlockZero to GetUnstoredBlock

[mtfishman]

Closes #4, closes #13.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.

Project coverage is 27.30%. Comparing base (abe5237) to head (c84a348).

Files with missing lines	Patch %	Lines
...ksparsearrayinterface/blocksparsearrayinterface.jl	0.00%	2 Missing ⚠️
src/blocksparsearrayinterface/getunstoredblock.jl	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   27.27%   27.30%   +0.03%     
==========================================
  Files          26       26              
  Lines         869      857      -12     
==========================================
- Hits          237      234       -3     
+ Misses        632      623       -9     

Flag	Coverage Δ
docs	27.21% <66.66%> (+0.03%)	⬆️

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