dataclass validation

[samuelcolvin]

Change Summary

Validation of dataclasses so they can be nested

Related issue number

fix #273

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes
• No performance deterioration (if applicable)
• HISTORY.rst has been updated
  • if this is the first change since a release, please add a new section
  • include the issue number or this pull request number #<number>
  • if you're not a regular contributer please include your github username @whatever

[codecov]

Codecov Report

Merging #334 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #334   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          13     13           
  Lines        1783   1855   +72     
  Branches      347    378   +31     
=====================================
+ Hits         1783   1855   +72