Layout tree text representation

Summary

The hardware layout tree needs a text representation that can be hand-edited. From the poll it is very clear that a mixed representation is favored.

I was at first considering JSON as the input format, but there are two caveats:

• Comments are not supported
• "Flat" representations are not supported natively and have to be implemented on top of the parser

For these reasons I'm now gearing towards YAML. It brings in another dependency, yaml-cpp (the library is great despite terrible lack of documentation). YAML natively supports anchors and type hints, so we could write eg:

%YAML 1.2
---
# The VFATs
.vfat-42: &vfat-42 !vfat
    id: abcd-efgh
    location: 0
.vfat-24: &vfat-24 !vfat
    id: ijkl-mnop
    location: 1

# This chamber is used elsewhere in the document with *chamber-123
.chamber-123: &chamber-123 !chamber
    id: GE11-CERN-0001
    location: 12
    children:
      - !optohybrid_fpga
        id: something here
      - *vfat-42
      - *vfat-24
...

Unfortunately YAML anchors can't work between multiple files.