Add option for automatic multicolumn header for groups

[cgnguyen]

Maybe this is related to this #3.

I find that using the groups label often makes the tables too wide, because the group names make the first column of the table very wide.

I get around this by manually changing the column command to a multi-column that spans the entire width of the table, but this way I have to manually edit every table. Is there a way to automate this?

[cgnguyen]

For reference - here is the difference in practice

Table too wide
https://imgur.com/a/gdruDlX

Condensed table
https://imgur.com/a/lKidJ4r

[leifeld]

Thanks. I guess this could be a useful addition. Let's treat this separately from #3.

One would need to wrap the group labels in a \multicolumn command. Without looking at the code, off the top of my head, I'm not sure if that's easy to add or if it would require some rewriting of the functions that generate the basic table layout. One should also check the ensure that the changes are fully compatible with the optional dcolumn argument.

If you'd like to give it a try, feel free to add a pull request with some suggested code changes and we'll review it. If you don't feel comfortable trying it, we'll get to it at some point after resolving other issues. Thanks!