rows can be 0 in virus panel grid calculation

[terrycojones]

Here's an exception:

Traceback (most recent call last):
  File "/scratch/tcj25/root/.virtualenvs/35/bin/proteins-to-viruses.py", line 108, in <module>
    sampleTotalReadCountFilename=args.sampleTotalReadCountFilename))
  File "/scratch/tcj25/root/.virtualenvs/35/lib/python3.5/site-packages/dark/proteins.py", line 319, in toHTML
    self.virusPanel(virusPanelFilename, sampleTotalReadCountFilename)
  File "/scratch/tcj25/root/.virtualenvs/35/lib/python3.5/site-packages/dark/proteins.py", line 709, in virusPanel
    figure, ax = plt.subplots(rows, cols)  # , squeeze=False)
  File "/scratch/tcj25/root/.virtualenvs/35/lib/python3.5/site-packages/matplotlib/pyplot.py", line 1186, in subplots
    ax0 = fig.add_subplot(gs[0, 0], **subplot_kw)
  File "/scratch/tcj25/root/.virtualenvs/35/lib/python3.5/site-packages/matplotlib/gridspec.py", line 154, in __getitem__
    raise IndexError("index out of range")
IndexError: index out of range
make: *** [index] Error 1

This happens when rows = 0 in the following (from proteins.py line 707):

        cols = 5
        rows = int(len(virusTitles) / cols) + (
            0 if len(virusTitles) % cols == 0 else 1)
        figure, ax = plt.subplots(rows, cols)  # , squeeze=False)

rows will be 0 when virusTitles is 0. Presumably we shouldn't have called virusPanel in this case.

This is all triggered by running

$ find . -name summary-proteins | proteins-to-viruses.py --html --virusPanelFilename viral-sample-read-counts.png --sampleNameRegex '^\./(RISE\d+)/' > index.html

so fix it in toHTML or similar.