rm picoseconds displayValue from signal-generator
DRY up model type-based model instantiation
begin sensor model by "forking" signal-generator

[#48362817]

Considering this wholesale copy-paste justified because the real purpose of signal-generator, so far, is to be a starting point for the sensor model. We'll eventually find the common bits.
Makefile changes for sensor model type

[#48362817]
Also fork "signal generator" interactives

[#48362817]
Borrow "mini-class.js" from IS experiment

[#48362817]
Import applet-handling code from ISImporter

[#48362817]
move sensor units-definition to its own file

[#48362817]
sensor definitions file

[#48362817]
sensorType -> measurementType

[#48362817]
Working version of sensor model

[#48362817]
Working 'sensor' interactive

[#48362817]