Get gene length and GC content of `gene` from data frame (`df`) containing gene IDs in a column labeled the value of `column_id`. `df` must have a column labeled sequence where the sequence info for exons/or transcripts are and feature start and stop coordinates in columns labled the value of `start` and `stop`. This will return a vector of `gene`, length, and GC content as a percentage.
biomart_stats(gene, column_id, df, start, end)a gene name corresponding to a value in `df[,column_id]`
the column name containg gene feature IDs
the dataframe of exon/transcript annotations
column name of feature start coordinate
column name of feature end coordinate