Introduce signal node object to abstract connections with control devices

[pelesh]

Description

This is a draft for implementation of signal node (bus) object.

Closes #154

@superwhiskers

Proposed changes

In this implementation signal node does not own any variable or residual. The SignalNode class implements methods to set, initialize and read signal:

1. void set(ScalarT* p_signal) method stores the pointer to a component model variable that will be shared with other components as a (control) signal. Only component that owns that variable can invoke the set function.
2. void init(ScalarT signal) writes value of signal to the memory location pointed by the pointer stored in the SignalNode instance. Both, the component sending and the component receiving signal can initialize the signal.
3. ScalarT read() is a const method that reads the value pointed by the pointer stored in the SignalNode instance.

Note: When initializing the system, generators need to be initialized before governors because mechanical power sent to a generator by the governor is initialized by the generator rather than the governor.

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.

Further comments

• Current implementation keeps the old interface where generator has pointer directly to the governor and vice versa.
• Still TODO: Create method to select which variable to connect to the signal bus.

[pelesh]

Some additional comments on ideas behind the proposal in this PR:

• Components that are source of signals own signal variables. I think this is helpful because:
  • The number of component internal variables is always the same no matter what is the connectivity.
  • Internal variables including signal variables can be stored in a contiguous array to simplify implementing compile-time automatic differentiation.
• Signal node stores address and/or index of the variable shared as a signal. The signal node has minimal functionality and its purpose is to abstract away the source of signal from component receiving the signal.
• Components that receive signal get the address/index of the signal variable so they can use it directly in its residual/Jacobian equations or make a local copy of the signal variable.

[lukelowry]

Overall I prefer this approach to #173 , but we still need to make the signals definable via data structure soon. Up to @pelesh if we do that in this PR or as next task.

[lukelowry]

All looks well. I prefer this over the initial PR for SignalBus #173

I believe merging this will allow me to submit PR for the exciter model in my feature branch. Hope we can get this approved asap!