Initial implementation of generic phasor dynamics app

[PhilipFackler]

Description

This is a start on a generalized phasor dynamics app.

Closes #349

Proposed changes

This PR introduces the executable target pdsim, the source for which is under GridKit/Apps/PhasorDynamics. It is based on the ThreeBusBasic example and includes event types relevant to that example using the proposed parameterized study input file from #349. I also added tests for this app based on the TwoBusBasic and ThreeBusBasic examples.

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.

[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).

[lukelowry]

This is a great addition and an important step, so I had a lot of feedback. I had enough edits that I opened PR #363 on this branch (apologies if I did that incorrectly).

I added various safety checks and extended the study format. Please let me know what you think and perhaps it can be merged into this PR. Thank you

[pelesh]

This is a great addition and an important step, so I had a lot of feedback. I had enough edits that I opened PR #363 on this branch (apologies if I did that incorrectly).

I added various safety checks and extended the study format. Please let me know what you think and perhaps it can be merged into this PR. Thank you

Thanks for the feedback @lukelowry, I responded in your PR.

[pelesh]

Looks good.

Please move directory GridKit/Apps --> application (same level as `GridKit dir). Applications are not part of the GridKit proper but rather built on top of GridKit library.