Disable subtotal line in groups - Up to date

[eagleoriginal]

Propose new feature. Subtotal Line totally removed if only merging needing.
This achieve by using tag DisableSubtotalLine. Like <<Group MergeLabels=Merge2 DisableSubtotalLine>>
Sometimes it can be useful, when users ask for groups in secondary columns(it can be date, or some entity), but subtotals usually needs only in first column(usually organization).
result you can see in file
GroupTagTests_DisableSubTotals_MergeLabels.xlsx

Also fix bug of style of merged cell(sometimes merged cells become gray).
Because in row on right column placed subtotal footer.
After that some tests have broken with error
for example ClosedXML.Report.Tests.GroupTagTests.Customers
TestCase InlineData("GroupTagTests_DisableOutline.xlsx")
Expected True because Cell style are not equal starting from B6, but found False.
I dont know much ClosedXml.Reports but there are something wrong in line:

int cellIdx = _maxLevel - subGroup.Level + 1;