Addition of residual block support for pyg_to_hls

[green-cabbage]

This is my first time submitting a pull request, let alone for work, so corrections are appreciated.

Description

• I have added a custom block called residualBlock on top of existing nodeblock, edgeblock and edge_aggregate.
• The nnet utils h file for this block is located on templates/vivado/nnet_utils/nnet_graph.h because I am implementing this on my group's GNN, but could more properly belong in templates/vivado/nnet_utils/nnet_merge.h
• There's yet no dataflow/pipeline distinction for residualBlock. Mathmatically it's a simple operation so there may not be a need to.

Type of change

• [x] New feature (non-breaking change which adds functionality)

[JanFSchulte]

This is an extension of now close PR https://github.com/fastmachinelearning/hls4ml/pull/379 and will therefore also be closed.