Decimals where unnecessary

[OssianEPPlus]

Multiple areas in Epplus use Decimal rather than double for no apparent reason even when excel uses doubles. A few examples are:

• ExcelChartTrendline.cs which uses it for many properties: Order, Period, Forward, Backward, Intercept

• ExcelDoughnutChart.cs that uses it in FirstSliceAngle, HoleSize

• ExcelTime.cs which uses it everywhere.

• ExcelWorkbook.cs Which uses it for MaxFontWidth

• ExcelView3D for perspective and rotation

• ExcelPrinterSettings for margins

• FontSize height and width

• ExcelSparklineColor and ExcelColorXml uses it for Tint.

Most or all of these would match excel and have less overhead if we used double instead as we do most other places in epplus.