Ignore subgraphs with only one node

[dstoc]

digraph G {
  foo;
  input [data="hello"];
  input -> log;
}

foo being present in the graph results in:

Unhandled rejection TypeError: Cannot read property 'edgesCount' of undefined
    at linearize (/home/dstockwell/local/erlnmyr/core/linearize.js:22:20)
    at Array.map (native)
    at linearConnectEdges (/home/dstockwell/local/erlnmyr/core/experiment.js:144:23)
    at buildstageList (/home/dstockwell/local/erlnmyr/core/experiment.js:175:16)
    at PhaseBaseRuntime.module.exports.doExperiment.definePhase.require [as impl] (/home/dstockwell/local/erlnmyr/core/experiment.js:291:36)
    at /home/dstockwell/local/erlnmyr/core/phase.js:387:28
    at runDependency (/home/dstockwell/local/erlnmyr/core/scheduler.js:200:5)
    at startTasks (/home/dstockwell/local/erlnmyr/core/scheduler.js:179:9)
    at done (/home/dstockwell/local/erlnmyr/core/scheduler.js:192:3)
    at /home/dstockwell/local/erlnmyr/core/scheduler.js:239:5
    at tryCatcher (/home/dstockwell/local/erlnmyr/node_modules/bluebird/js/main/util.js:26:23)
    at Promise._settlePromiseFromHandler (/home/dstockwell/local/erlnmyr/node_modules/bluebird/js/main/promise.js:503:31)
    at Promise._settlePromiseAt (/home/dstockwell/local/erlnmyr/node_modules/bluebird/js/main/promise.js:577:18)
    at Promise._settlePromiseAtPostResolution (/home/dstockwell/local/erlnmyr/node_modules/bluebird/js/main/promise.js:244:10)
    at Async._drainQueue (/home/dstockwell/local/erlnmyr/node_modules/bluebird/js/main/async.js:128:12)
    at Async._drainQueues (/home/dstockwell/local/erlnmyr/node_modules/bluebird/js/main/async.js:133:10)