Pending confirmation: Try to fix single dataSet with custom groupSpace issue #284

[liuxuan30]

for issue #284

How to test the difference:
Force groupSpace to return custom value or 0.8 like below:

    /// The spacing is relative to a full bar width
    public var groupSpace: CGFloat
    {
        get
        {
            if (_dataSets.count <= 1)
            {
                return 0.8
            }
            return _groupSpace
        }
        set
        {
            _groupSpace = newValue
        }
    }

[danielgindi]

@PhilJay do you see any possible sideeffects to this?

[liuxuan30]

Why it forces to return 0.0 if only up to 1 dataSet. Is it allowed to return other value instead of 0.0 for this scenario?
I am not sure if it makes sense to return positive values. If not, then we can close it. I got this issue by QA testing.

[liuxuan30]

@danielgindi I am occupied with some dev work recently, I will update as soon as I got some time.

[liuxuan30]

get some time to merge with master. The only problem still remains:

Why it forces to return 0.0 if only up to 1 dataSet?
Is it allowed to return other value instead of 0.0 for this scenario?
I am not sure if it makes sense to return positive values. 
If not, then we can close it. I got this issue by QA testing.

[liuxuan30]

Today I have been thinking about this, groupSpace should be used when dataSets count > 1. Only one dataSet and setting groupSpace seems not make sense?

Besides, I found a bug that when trying to highlight the bars, the xIndex is not correct, due to the groupSpace is 0.8, not 0. Using 0.0 is correct highlighting. So I am thinking there must be some reasons to separate them. Closing this PR right now.