combines multiple xvg data objects into a single structure, preserving metadata from the first object and adding a group identifier to track the source of each data point.
merge_xvg_data(xvg_data)
a merged xvg data object with:
data - Combined data frame with an additional 'group' column identifying the source
metadata - Metadata from the first object in the list
a list of xvg data objects, each containing 'data' and 'metadata' components