gen_and_contaminate_reads: Generate contaminated reads
In philliplab/MotifBinner: Bin reads by Motif

Basically calls gen_reads twice and mixes the results together

1 2	gen_and_contaminate_reads(ref_seq, n_reads, error_rates, contam_seq, n_contam, seed = NULL, ...)

`ref_seq`	The true sequence from which to generate the 'src' reads
`n_reads`	The number of reads to generate
`error_rates`	The error profile as generated by gen_error_profile
`contam_seq`	The sequence to generate the contaminated reads from
`n_contam`	The number of contamination reads to include
`seed`	The seed for the randomizer