More functionality for SparseArrayDOK

[mtfishman]

This builds on #3.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 69.10%. Comparing base (0facc75) to head (9436feb).
Report is 1 commits behind head on derive.

Files with missing lines	Patch %	Lines
src/sparsearrayinterface.jl	81.81%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           derive       #4       +/-   ##
===========================================
+ Coverage   31.66%   69.10%   +37.43%     
===========================================
  Files           4        5        +1     
  Lines         120      123        +3     
===========================================
+ Hits           38       85       +47     
+ Misses         82       38       -44     

Flag	Coverage Δ
docs	68.59% <86.66%> (+38.08%)	⬆️

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