Add _in_construct_mode to differentiate container init vs construct

[rly]

Motivation

Fix #750. This PR adds a new single-underscore-protected field in AbstractContainer called _in_construct_mode that is set to True if the object is being created by the ObjectMapper when constructing the object from builders read from a file and then reset to False after the object is done being created. Users creating the object will not interact with this field.

Subclasses of AbstractContainer can check _in_construct_mode to determine whether to raise a warning or error when encountering invalid data (we want to be able to read and fix data that is invalid but not create new data that is invalid).

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #751 (37505cd) into dev (a476142) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #751   +/-   ##
=======================================
  Coverage   87.57%   87.57%           
=======================================
  Files          44       44           
  Lines        8834     8836    +2     
  Branches     2033     2033           
=======================================
+ Hits         7736     7738    +2     
  Misses        784      784           
  Partials      314      314           

Impacted Files	Coverage Δ
src/hdmf/build/objectmapper.py	92.73% <100.00%> (+<0.01%)	⬆️
src/hdmf/container.py	90.14% <100.00%> (+0.01%)	⬆️

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