at – 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 sum &gt; 0?
    * verbose printing
    &#34;&#34;&#34;
    start = 0

    if verbose:
        start = time.time()
        print(&#34;Preprocessing - running remove_non_expressed ... &#34;, end=&#39;&#39;)
    
    mask = (df.sum(axis=1) / df.shape[1]) != 0

    df_filtered = df[