Remove spaces in filenames generated by nbconvert

If the a notebook has spaces in its filename (e.g., "x and y.pynb") then the generated files have spaces in them, such  "x and y_files/x and y_10_0.png".  This breaks \includegraphics in LaTeX which can't cope with spaces in filenames.  This patch ensures that filenames don't have spaces by replacing them with underscores.  Thus generated files have names like "x_and_y_files/x_and_y_10.0.png".  The generated .tex file itself still has spaces.

The patch is a simple modification of the unique_key and output_files_dir, and some tests.  "tests/files/notebook1.ipynb" is copied and renamed to "tests/files/notebook with spaces.ipynb"