Multi-head model support #149

[przemyslaw-aszkowski]

TODO:

• [x] Remove the clip for the segmentation model in the postprocessing function
• [x] Allow for multi outputs for regressor and segmentor
• [x] Fix get_channel_name due to changes
• [x] Fix _create_result_message due to changes
• [x] Update docs
• [ ] Add an example for a multi-head model for segmentation
• [ ] Add an example for a two-class model for object detection
• [x] In-depth evaluation

[bartoszptak]

@przemyslaw-aszkowski I think the PR is ready for review and merge.

Edit: Nope, there is something wrong with export features.

[bartoszptak]

Now works fine!