Initial implementation of multi-contingency app

[PhilipFackler]

Description

Refactored pdsim code as ContingencyStudy class and reused in initial implementation of concurrent contingency app.

Closes #392

Proposed changes

For the work-in-progress I implemented three different versions of the simulation for easy timing comparison.

• serial
• async (using std::async)
• openmp

The easiest interesting case to try locally is "new england" (I have 22 cores in my laptop).

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.

[lukelowry]

'ContingencyStudy' -> 'CTGStudy'

CTG is common short hand and way easier to type! One option.

[PhilipFackler]

yes, I need feedback on naming things

[pelesh]

A few comments:

• Great to have multiple execution options to compare performance!
• I suggest restoring PDSim as it was. It is a different app and should not be modified in this PR.
• I would contain implementation of ContingencyStudy to a single file or a source and a header file.
• We have development of application utilities in our roadmap, but this is beyond the scope of this PR.