Remove tomopy example and dependency.

[danielballan]

Tomopy has broken the build. It looks like this will take significant effort to
resolve, so I would like to un-break the docs now by removing tomopy and then
later (hopefully soon) restore the tomopy example once we figure out how to
build it.

[danielballan]

See #112 for my work in progress to revert this and get tomopy working again.

[mrakitin]

Let's use it as a temporary solution before we figure out how to compile tomopy in #112.

[danielballan]

Great, thanks.