Linked inputs test/figure infrastructure

[Roly Perera]

• Migrate motivating-example.fld to linked inputs example
• loadLinkFig but for linked inputs
  • evalGC for the program in original environment – deal with as part of #780
  • Some kind of “environment selector” that applies a δv to particular entry in environment
  • Dual of fwd to compute outputs that depend on selection on input 1
  • bwd to compute related inputs
  • Reuse splitDefs

New “linked inputs” test/figure infrastructure, plus illustratie test case with appropriate expectation on related inputs (other parts of dataset/program).

Done/dropped:

• Remove redundancy between Benchmark.Main and Test.Main
• Rename existing link infrastructure to linkedOutputs
• New TestLinkSpec variant for linked inputs
• LinkFigSpec, LinkFig variants for linked inputs
• Reinstate average-series.fld slicing test
• Placeholder for BubbleChart datatype
• Initial bubble-chart.fld example
• linkedInputs_cases with single case loading bubble-chart example

Blocking on:

• #779