ne expression data, e.g. UMI counts.
    
    verbose : bool, optional (default: False)
        Print progress report.
    
    Returns
    -------
    df_filtered : DataFrame
        Filtered data with no genes with mean 0 expression across all cells.

    NOTE:
    * df format – rows: genes, cols: cells.
              A     B     C
        POMC  0.0   0.5   0.9
        AGRP  0.2   0.0   0.0
        LEPR  0.1   0.1   0.4

    TODO:
    * consider a faster way of determining this. Perhaps just check if su