Initial implementation of generic phasor dynamics app

[PhilipFackler]

Description

Please describe the issue that is addressed (bug, new feature,
documentation, enhancement, etc.). Please also include relevant motivation and
context. List any dependencies that are required for this change.

See #349

Proposed changes

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should be accepted.

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• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
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• There are unit tests for the new code.
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• The feature branch is rebased with respect to the target branch.
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Further comments

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why you chose the solution you did and what alternatives you considered, etc...

[pelesh]

Looks good as is. A couple of things to address before we merge:

• Please add a README file explaining solver configuration input. A skeletal document is sufficient as I expect @abirchfield will want to add more features to that.
• Please make sure CI tests pass (see comment below).

[pelesh]

The GridKit code should build without SUNDIALS. Consider adding this guard to prevent failing CI tests:

Suggested change

	add_subdirectory(PhasorDynamics)
	if(TARGET SUNDIALS::idas)
	add_subdirectory(PhasorDynamics)
	endif()

[pelesh]

Perhaps call it solver or config?

Suggested change

	COMMAND PDSim ${CMAKE_CURRENT_SOURCE_DIR}/ThreeBusBasic.study.json
	COMMAND PDSim ${CMAKE_CURRENT_SOURCE_DIR}/ThreeBusBasic.solver.json

Similarly, we could rename ThreeBusBasic.json --> ThreeBusBasic.case.json to be more self-explanatory to the domain folk.