Remove _class_default from containers

[fwkoch]

Using empty containers as defaults caused big problems and was not consistent with the rest of properties (see #169). This PR is not backwards compatible, but I think it is the right choice.

[codecov]

Codecov Report

Merging #219 into dev will increase coverage by 0.04%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##              dev     #219      +/-   ##
==========================================
+ Coverage   95.03%   95.08%   +0.04%     
==========================================
  Files          15       15              
  Lines        2114     2114              
==========================================
+ Hits         2009     2010       +1     
+ Misses        105      104       -1

Impacted Files	Coverage Δ
properties/base/containers.py	88.75% <90.9%> (+0.29%)	⬆️

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