Extract _init_class_columns for use by DynamicTable subclasses

[rly]

This PR extracts code in DynamicTable.__init__ to DynamicTable._init_class_columns so that it can be called by subclasses of DynamicTable. This is necessary for the fix in NeurodataWithoutBorders/pynwb#1205

[codecov]

Codecov Report

Merging #323 into dev will increase coverage by <.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##              dev     #323      +/-   ##
==========================================
+ Coverage   72.73%   72.73%   +<.01%     
==========================================
  Files          33       33              
  Lines        6436     6438       +2     
  Branches     1411     1411              
==========================================
+ Hits         4681     4683       +2     
  Misses       1327     1327              
  Partials      428      428

Impacted Files	Coverage Δ
src/hdmf/common/table.py	61.87% <66.66%> (+0.17%)	⬆️

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