Make PeripheralTriangleMeshGenerator take multiple inputs

[YaqiWang]

Motivation

Currently PeripheralTriangleMeshGenerator can take a core mesh as the input and add unstructured mesh surrounding the core. However, in cases, there could be separated components like control drums, instrumentation, etc. outside of the core but disconnected with the core. It is thus desired that the mesh generator can take multiple inputs representing the core and those other components.

Design

The mesh generator is creating a sub mesh generator in XYDelaunayGenerator, which can already accept multiple holes. So adding this feature requires minor changes to the mesh generator. I will submit a PR for this.

Impact

A new capability.