Fix decimals crash in ChartsUtil

[pmairoldi]

Closes #1550, #1540, #1491

I added some tests based on the crash mentioned in #1550 and there was a crash when using DBL_MAX as the input of the decimals function.

There are still some abnormalities in the decimals function I believe. In MPAndroidCharts I ran the same tests and it returns 2 not zero for all these cases except for the ones that crash and return negative Int max.

This pull request fixes the crash but I don't think the decimals function does what we want it to do.

@danielgindi, @PhilJay what is this function actually supposed to do?

[danielgindi]

It's supposed to calculate a default number of decimals to show after the decimal point

[danielgindi]

It's not really important what the value is on edge cases. It is used as a default for formatters that were not configured by the user, it just shouldn't crash :-)

[pmairoldi]

The main reason I was asking what it does is that I believe it does not do what it's supposed to. See the testDecimalWithNormalValue test case. Shouldn't it return 6? If not then why are we doing some complex math if it can only be 0 or 1.

[umairnow]

This issue is still there in AxisRendererBase.swift on line 125
fatal error: Double value cannot be converted to Int because it is either infinite or NaN

[acegreen]

@petester42 @danielgindi this isn't resolved yet..

        let yRangeBuffer: Double = 50000
        lineChartView.xAxis.axisMinimum = set1.yMin - yRangeBuffer
        lineChartView.xAxis.axisMaximum = set1.yMax + yRangeBuffer

also crash with something as simple as:

lineChartView.xAxis.spaceMin = yRangeBuffer
lineChartView.xAxis.spaceMax = yRangeBuffer

[danielgindi]

Guys, the reason that this is not fixed, is that we do not see this error.
So you would have to:

1. Make sure you are doing everything correctly
2. Create steps for us to reproduce
3. Maybe do a little debugging. Why does a NaN or Infinity get passed to that function?