Assume parameters are input unless explicitly specified as output (#42)
Assume parameters are input unless explicitly specified as output (#42)

According to the [SlicerExecutionModel XML schema documentation](https://www.slicer.org/wiki/Documentation/Nightly/Developers/SlicerExecutionModel#XML_Schema), the `<channel>` parameter is only "required for file, pointfile, directory and image parameters". Examples make it clear that `<channel>input</channel>` is the intended default.

`slicer_cli_web` is currently too strict, requiring specifying the channel for all index parameters, regardless of type. This PR brings it to the other side, letting any parameter leave its channel unspecified for a default of `input`. This already appears to be the behavior for optional parameters.
girder/slicer_cli_web,girder/slicer_cli_web