set_xscale() and set_yscale() are called with arguments only if scale is logicle

[castillohair]

Fixes #289.

[castillohair]

This branch had to be based off contour-scikit-image instead of develop because it is impossible to test without the changes therein. If #288 is merged into develop, the section "files changed" will look as intended. I recommend doing #288 first.

[JS3xton]

Can this script be incorporated as a unit test? (not checking the output of the plot, just asserting that the call doesn't throw an error)

[castillohair]

I think tests are outside the scope of this issue, but I opened a new issue addressing this topic more generally in #292.