Add ability to specify height of a group

[tylersayshi]

Hi! This is hopefully a relatively simple PR. I am needing to set the height of my row groups as a static number of pixels. I tried setting the mode to 'fixed' and then styling the height of the row but this causes many other issues.

I'd be happy to update docs as needed or tweak the MR however is needed, but I would like to add this ability to specify the actual height of rows.