Bug 28187: Exclude dtrg-group from row striping.

Bug 28187: Exclude dtrg-group from row striping.

The odd row lowlighting stripe effect added to datatales wasn't
accounting for classes introduced by the rowGroup plugin to denote
grouping header rows. This patch adds the required exclusion rule to
properly highlight rowGroup grouping rows.

Test plan
1/ Enable POS
2/ Create a register and make some transactions.
3/ Have both debits and credits
4/ Have some older tranactions so you can look at the past_sales table.
5/ Apply patch and regenerate the staff CSS:
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_interface)
6/ Look at the sales and past_sales table.
7/ Make sure each row header is the darker shade of gray (#e0e0e0)
7a/ You will need to have made some transactions with even and odd
numbers of items in the sale highlight the problem before applying the
patch
7b/ The rowGroup plugin is also used for the checkouts table and
this patch should also fix that case.

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>