Rename variables in a table.
Renames selected variables in the table obj based on the mapping provided in renameMap.
renameMap is an n-by-2 cellstr array, with the old variable names in the first column, and the corresponding new variable names in the second column.
Variables which are not included in renameMap are not modified.
It is an error if any variables named in the first column of renameMap are not present in obj.