Make MAF output optional

[charles-plessy]

Description of feature

I see this in another pipeline:

            [
                path: { "${params.outdir}/QC_longreads/NanoLyse" },
                mode: params.publish_dir_mode,
                pattern: "*.log"
            ],
            [
                path: { "${params.outdir}/QC_longreads/NanoLyse" },
                mode: params.publish_dir_mode,
                pattern: "*_nanolyse.fastq.gz",
                enabled: params.save_lambdaremoved_reads
            ]

We could use the same way to make the output of maf.gz files optional (they are large).