ConstantSignalSource component and example with BusToSignalAdapter

[PhilipFackler]

Description

Introduce new component model as a simple example of sending currents to buses via the bus signal adapter

Proposed changes

ConstantSignalSource takes complex parameter and makes it available on output signal nodes (real and imag).

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• [p] 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]

I suggest we document this better and mark it ready for review. @lukelowry might have additional suggestions how to make the usage example more relevant for domain experts.

[pelesh]

There are issues with how signals are handled in GridKit that are inherited in this PR but are out of scope here (see #407).

For now please rebase and we can merge this PR.