Improve MultiQC call and explanation in RNAseq mini-course

[vdauwera]

Excellent suggestions from Ken for the MultiQC code and explanation:

This bit first:

        FASTQC.out.zip.mix(

        Channel.empty.mix(
            FASTQC.out.zip,

Then this bit to break down some of the complexity

    // Comprehensive QC report generation
    multiqc_files_ch = Channel.empty.mix(
        FASTQC.out.zip,
        FASTQC.out.html,
        TRIM_GALORE.out.trimming_reports,
        TRIM_GALORE.out.fastqc_reports,
        HISAT2_ALIGN.out.log
        )
    multiqc_files_list = multiqc_files_ch.collect(),
    MULTIQC(
        multiqc_files_list,
        params.report_id
    )

Originally posted by @kenibrewer in #526 (comment)