Improve semantics of data logging

[psychemedia]

At the moment the data logging and charting is handled by using a print() statement to print sensor log values to the the display window using statements of the form print('Light_left: ' + str(colorLeft.reflected_light_intensity)).

These values are then used to update:

• the live data log chart display (if it is enabled) via outf() in templates/studio.js;
• the Ev3DevWidget.results_log in ev3devsim_nb.py; this object is itself updated via the report_callback() function defined via Ev3DevWidget.read() and called from within the outf() function in templates/studio.js.

It might be more convenient to define a Skulpt function that we can use to log sensors rather more formally and naturally. For example, in something like a Skulpt src/lib/simlogger.js. Does the following provide a start? (I'm not sure how best to define either the py interface, or the Javascript function that handles it.)

function datalog (channel, obj) {
    //obj.$jsstr()
    //???
  }

var $builtinmodule = function(name)
{
    var mod = {};

    mod.datalog = new Sk.builtin.func(function(channel, obj){
        datalog(channel, obj);
        return new Sk.builtin.none;
    })

    return mod;
}

//from simlogger import datalog
//Maybe something like:
//datalog("left_light", colorLeft.reflected_light_intensity)