Update to full C++11 standard

[ischoegl]

Changes proposed in this pull request

• Update compiler versions in CONTRIBUTING.md
• update compiler specification in SConstruct (replace -std=c++0x by -std=c++11)
• add recommendations for whitespace use ... created a separate PR (Add recommendations for whitespace use to CONTRIBUTING.md #1121)

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

Thoughts

• MSVC uses C++14 by default (can be specified via /std:c++14 after Visual Studio 2015 Update 3 - see here)
• Not sure whether the Cygwin work-around (-std=gnu++11) is still necessary ... probably yes

[codecov]

Codecov Report

Merging #1119 (f7d5e9a) into main (be95118) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1119   +/-   ##
=======================================
  Coverage   73.45%   73.45%           
=======================================
  Files         364      364           
  Lines       47882    47882           
=======================================
  Hits        35170    35170           
  Misses      12712    12712           

Impacted Files	Coverage Δ
src/thermo/PengRobinson.cpp	88.26% <100.00%> (ø)

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[speth]

Works for me.