Check for LogNorm with imshow and export correctly

[OliverZobel]

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Matplotlib figures created with imshow and a LogNorm normalization do not get exported correctly, i.e. the log scale is ignored. This pull request changes the code to check for a logarithmic normalization and, if found, applies np.log() to the data and clims in the plt.imsave() call in order to produce a correct .png image.