Improve sizing of figures in the PDF output

[thomaspiller]

LaTeX does not interpret image dimensions as defined for the HTML output. This results in many images being too large and lacking definition in the PDF.

One approach to solve this is to use conditional blocks to define different settings for HTML and PDF outputs:

.. only:: html

   .. image:: path/to/your/image.png
      :width: 50%

.. only:: latex

   .. image:: path/to/your/image.png
      :width: 10cm

This was implemented on part of the User interface chapter (b59d71f) and should be extended to the whole documentation.