Ambiguous Determination of what arguments of the functions in workflows are images

[Cryaaa]

Since the sources_of function assumes that all arguments of a function which are strings are images this might cause problems with functions which have other strings as argument values (i. e. for selecting an algorithm or similar). It should be possible to save with arguments are images when setting the workflow step since at this step the user knows which arguments are images.

[haesleinhuepf]

I see two ways for doing this:

• Use the signature of a given function and check if parameters are supposed to be images. See this code.
• Alternatively, we can check if a parameter value is of type string and then check if the string equals to any layer name. See that code.