add support for legacy SDK

[tlambert03]

edit
though this doesn't actually "integrate" the legacy SDK into the main ND2File object... I'm merging at this point because i want this in main so I can work on smaller bits of functionality in smaller PRs

[codecov-commenter]

Codecov Report

Merging #2 (3da703a) into main (4bbc1be) will not change coverage.
The diff coverage is 0.00%.

@@          Coverage Diff           @@
##            main      #2    +/-   ##
======================================
  Coverage   0.00%   0.00%            
======================================
  Files          4       5     +1     
  Lines        303     498   +195     
======================================
- Misses       303     498   +195     

Impacted Files	Coverage Δ
nd2/__init__.py	0.00% <0.00%> (ø)
nd2/_chunkmap.py	0.00% <0.00%> (ø)
nd2/_util.py	0.00% <0.00%> (ø)
nd2/nd2file.py	0.00% <0.00%> (ø)
nd2/structures.py	0.00% <0.00%> (ø)

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