Add high-level support for communication through ACCL

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• Adds ACCLOut and ACCLIn nodes which send data across FPGAs by invoking ACCL
  • Currently only point to point communication is supported, but other types of communication might be added
• Supports verification with cppsim using ACCLs BFM emulator
• I planned two follow-up PRs
  • Integrate finn-experimental partitioning and extending the build flow to support distributed designs
  • Adding low-level support for synthesizing designs with ACCL (in conjunction with the work being done on integrating FINN with Coyote)