Added Errorbars and fixed an issue

[LanPodder]

Errorbars can be added to any line or combo chart other than 3d charts and bubble charts using this:

chartData = [
    {
        name: 'Some name',
        labels: ['columnone', 'columntwo'],
        values: [value1, value2],
        errorrate: [errvalue1, errvalue2]
    },
    {
        name: 'Some other name',
        labels: ['columnone', 'columntwo'],
        values: [othervalue1, othervalue2],
    }
]
slide.addChart(chartData, opts)

Errorbars are optional on every chart. you can add a combo chart with 4 lines/bars and have only one of them include Errorbars. Currently does not support standard errorbars. Does not support further customization of errorbars (colors etc)

Deals with #652

[MariusOpeepl]

This feature would be really nice to have; is there anything I can help with on this, @gitbrent?

[gitbrent]

It's marked for 3.x, so it's on the horizon.

The diff shows about 240 changes to gen-charts.ts, so it's not very easy to review right off-hand.

[jadzeino]

can we please add this feature

[LanPodder]

Bump, resolved merge conflicts