Adding SimpleBlock to pyomo.core namespace

[whart222]

Fixes NA.

Summary/Motivation:

. Add SimpleBlock to the pyomo.core namespace

Changes proposed in this PR:

• We expose SimpleVar and related objects to simplify the
  API for pyomo.core users. This change adds SimpleBlock to be
  consistent.
• This allows Pyomo extension packages to import SimpleBlock from pyomo.core, instead of from a module in pyomo.core. This enhances the stability of Pyomo extension packages, since Pyomo can change its internal organization without impacting extension packages that pull symbols from pyomo.core.

Legal Acknowledgement

By contributing to this software project, I agree to the following terms and conditions for my contribution:

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.

[codecov-io]

Codecov Report

Merging #941 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #941   +/-   ##
=======================================
  Coverage   73.46%   73.46%           
=======================================
  Files         469      469           
  Lines       71514    71514           
=======================================
  Hits        52541    52541           
  Misses      18973    18973

Impacted Files	Coverage Δ
pyomo/core/base/block.py	83.88% <ø> (ø)	⬆️

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