PDF documentation on readthedocs broken due to image file names

[nickerso]

Some of the image file names in the libCellML documentation is causing the PDF generation on readthedocs to fail:

...MLRoughObjectModel-UseCases1.i-1.iii.png}

! LaTeX Error: Unknown graphics extension: .i-1.iii.png.

the HTML version is unaffected. Renaming the files and updating the references should be sufficient to avoid this issue.

[nickerso]

Actually, the build status reports failure on readthedocs, but the actual PDF is still produced and looks fine - apart from missing the image with the "unknown extension".

easy link: https://media.readthedocs.org/pdf/libcellml/latest/libcellml.pdf