Some error bar clean ups

[etpinard]

@alexcjohnson @mdtusz @cldougl @bpostlethwaite

This PR:

• fixes default logic for error bar type: 'sqrt'
• adds test for error bar type: 'sqrt' (about time)
• reuse svg error bar logic in 3d convert step
• split error bar default, calc and reusable compute-error-magnitude routine into separate files
• remove an used function

[etpinard]

reusable error bar data/constant/sqrt/percent logic

[alexcjohnson]

Looks good to me. Don't need to do it now, but it occurs to me we could boost performance by just evaluating opts once and returning a more minimal function to evaluate in the loop:

function computeError(opts) {
    if(type === 'data') {
        if(opts.symmetric || opts.arrayminus === undefined) {
            var array = opts.array
            return function(dataPt, index) {
                var val = +(array[index]);
                return [val, val];
            }
        }
        // etc...
    }
}

[etpinard]

good call.

[etpinard]

all about the 🐎

[alexcjohnson]

Thanks @etpinard , 💃 for my part.

[etpinard]

@bpostlethwaite @mdtusz better?

[etpinard]

@alexcjohnson is this what you had in mind?

[alexcjohnson]

😍

[mdtusz]

Seems like an ideal place to use a switch statement.

[etpinard]

We've been reluctant to use swtich statements in plotly.js so far. Maybe this is the time to use them more.

@bpostlethwaite switch statements are kinda messy in js, are they considered a good pattern?