Adding an option to ConfigBase.__call__ to preserve implicit values

[jsiirola]

Summary/Motivation:

When using ConfigBlocks for configuring complex objects (like solvers in Pyomo), it becomes convenient to create temporary ConfigBlocks for holding transient options. We do that by calling the ConfigBlock. The catch is that if the block permits implicit keys, then we will want to preserve those implicit keys.

This PR adds a new option to __call__ to cause the copy to preserve all implicitly-defined keys.

Changes proposed in this PR:

• add preserve_implicit option to ConfigBase.__call__()

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[coveralls]

Coverage increased (+0.004%) to 64.004% when pulling f910407 on configblock-implicit-duplication into ea05379 on master.

[codecov-io]

Codecov Report

Merging #45 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   64.92%   64.94%   +0.01%     
==========================================
  Files          87       87              
  Lines        8725     8726       +1     
==========================================
+ Hits         5665     5667       +2     
+ Misses       3060     3059       -1

Impacted Files	Coverage Δ
pyutilib/misc/config.py	94.96% <100%> (ø)	⬆️
pyutilib/misc/comparison.py	88.56% <0%> (+0.36%)	⬆️

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[blnicho]

This looks fine.