Json stringify object measurements

This change will e.g. make the values of `typing.NamedTuple`s visible in the Web UI. Before this change they were displayed as `[object Object]`.
I believe the measurements are guaranteed to be json-serializable because they are coming in via the websocket transport, which uses json already.

openhtf/output/web_gui/src/app/stations/station/station-data.ts

@@ -189,7 +189,9 @@ function makePhase(phase: RawPhase, running: boolean) {
     measurements = Object.keys(phase.measurements).map(key => {
       const rawMeasuredValue = phase.measurements[key].measured_value;
       let measuredValue = null;
-      if (typeof rawMeasuredValue !== 'undefined') {
+      if (typeof rawMeasuredValue === 'object') {
+        measuredValue = JSON.stringify(rawMeasuredValue);
+      } else if (typeof rawMeasuredValue !== 'undefined') {
         measuredValue = `${rawMeasuredValue}`;
       }
       return new Measurement({