Update ConfigValue display() to produce prettier output

[jsiirola]

Fixes: N/A

Summary/Motivation:

Update ConfigBlock display() so that complex data types (like pyomo's _PathData objects) are converted to strings before printing them out (instead of dumping them as yaml). This should also speed up the ConfigBlock system, as walking the data hierarchy no longer generates strings.

Changes proposed in this PR:

• Update display() to produce "prettier" output

Legal Acknowledgement

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[coveralls]

Coverage decreased (-1.02%) to 61.055% when pulling 64db3a8a09c0107dfa96bd13f3fbb216badd745e on config-display-update into 5763915b62477c911cba2eae4de227a97171fe8d on master.

[codecov-io]

Codecov Report

Merging #76 into master will increase coverage by 0.05%. The diff coverage is 87.75%.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
+ Coverage   62.85%   62.91%   +0.05%     
==========================================
  Files          87       87              
  Lines        8777     8804      +27     
==========================================
+ Hits         5517     5539      +22     
- Misses       3260     3265       +5

Impacted Files	Coverage Δ
pyutilib/misc/config.py	94.28% <87.75%> (-0.53%)	:arrow_down:

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