update neo_rawio datatypes

[samuelbray32]

Resolves #98

• add null buffer value to instances of _signal_stream_dtype and _signal_channel_dtype
• set minimum version of neo with the new data format

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.32%. Comparing base (f6399c8) to head (5fc1832).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #99   +/-   ##
=======================================
  Coverage   90.32%   90.32%           
=======================================
  Files          25       25           
  Lines        2429     2429           
=======================================
  Hits         2194     2194           
  Misses        235      235           

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