Resolve PlugFlowPipe compatibility with PartialTwoPortInterface

[bravache]

Linked issue (with further discussion): lbl-srg/modelica-buildings#2449

Issue

The IBPSA.Fluid.FixedResistances.PlugFlowPipe model extends from PartialTwoPortVector, which uses a vectorized outlet port. This inheritance is at odds with the other pipe models in the FixedResistances packages, and prevents this model from being used to redeclare PartialTwoPortInterface objects.

Context

The model IBPSA.Fluid.FixedResistances.PlugFlowPipe is a wrapper for the IBPSA.Fluid.FixedResistances.BaseClasses.PlugFlowCore that incorporates a mixing volume vol and a vectorized outlet port ports_b[].

The mixing volume is used to model the heat capacity of the pipe walls and the outlet port is vectorized to prevent the need to add a splitter or mixing volume after the model when the outlet is branching out, therefore making the integration of the model more computationally efficient.

Proposed solution

The solution proposed is to deprecate the PlugFlowPipe model as it currently exist, and promote the BaseClasses.PlugFlowCore in its place. The mixing volume would either be integrated into the PlugFlowCore model, but connected to a non-vectorized port (similar to the implementation of Buildings.Fluid.FixedResistances.Pipe in MBL) or removed entirely (if we can validate that its impact on outputs are negligible in most use cases).

This solution would simplify the use of the plug flow model, and make its integration more explicit and clear (i.e. the model only represent a pipe, and not a pipe + mixing volume). This solution also remove the redundancy of having two models with similar usage, name and behavior (PlugFlowPipe and BaseClasses.PlugFlowCore).