Bar chart custom value offset

[bivant]

Issue Link 🔗

Vertical version the HorizontalBarChartView issue #124
Possibly (in future) #777

Goals ⚽

Scope: BarChart (no horizontal bar logic for now)
Add ability to specify custom value offset for a value label (prior value is 4.5, what is still true by default)
When was implementing this feature I faced with the issue with low bars - text was unreadable when it has the same color as the background. So secondary value colors were added. After that I realized that inverting the value text color would be more universal and require almost no additional logic (except stacked, but that was later), so added this feature also.
Although coloring logic can be a separate PR, I have no interest in it without custom/flexible value positioning, so please not send me to split it right away.
While I need a non-stacked version I saw that drawing logic is very similar to "single bar" so added support of the features to it as well.

Implementation Details 🚧

BarChartDataProvider (affecting BarChartView):

    /// if set to true values those
    /// 1.do not fit into the value bar are drawn above them (below for negative), instead of partially inside and/or below (above if negative)
    /// 2.do not fit into the visible area above/below their bars are drawn inside
    var isDrawValueSideFlexible: Bool { get }
    /// distance from top (bottom in negative) for values drawn outside/inside the bar
    var valuesOffset: CGFloat { get }

BarChartView valuesOffset for a value label has same default value as before (4.5).
Setting a isDrawValueSideFlexible value resets the offset of vertical bars to zero as the padding is not needed because of the flexible position

ChartBaseDataSet:

    /// valueColors in case of soft positions in effect (BarCharView:isDrawValueSideFlexible)
    open var valueColorsSecondary = [NSUIColor]()

    /// Check if colors for the values for the DataSet are distinct from the background/bar color and apply inverted color if opposite
    open var valueColorsAdjustment: Bool = false

valueColorsSecondary are applied for the values affected by isDrawValueSideFlexible only.
valueColorsAdjustment in effect for all value labels in the chart if enabled

Testing Details 🔍

Tried to add tests for all the edge cases I could think of: custom value offset, flexible positioning, colors inverting+secondary, combination of them. Stacked and non-stacked versions.

[bivant]

I am ready to port the changes to the HorizontalBar if they are accepted.
These issues may be fixed:
#709
#124

For the #777 need guidance where to put the parameters as there should be a lot of them - position factor, offset, rotation, alignment, maybe something else.

[bivant]

Seems like the reference images for the new tests need to be updated, that was done for existing tests in master already after the merge with 4.0 branch.