Reformat a phyloseq object of qSIP_atom_excess analysis
qSIP_atom_excess_format(physeq, control_expr, treatment_rep)
A phyloseq object
An expression for identifying unlabeled control samples in the phyloseq object (eg., "Substrate=='12C-Con'")
Which column in the phyloseq sample data designates replicate treatments
numeric value: atom fraction excess (A)