docs: Added a detailed schematic of the pixel classification top-level operator.
ability to use externally pre-computed features for classification

This commit adds a new button (Precomputed Features...) to the feature
selection applet. As an alternative to selecting features from the
pre-determined set of features using the feature selection dialog, you
can now specify a text file which contains a list of features you have
pre-computed and stored on disk.

After clicking on the "Precomputed Features" button, select a text file
with the following format:

  - one filename per line
  - each filename is an HDF5 file, with dimensions xyz, no channels
    dataset is called "data"

You can then use these features for learning and prediction.
slightly nicer latex options for documentation
sanity checks
make feature from file actually work

This line took me at least a day to debug:
++j in python just does nothing!
++j in C/C++ increments by one
allow separate colors for drawn label and its probability
vigra watershed workflow: load .ilp file given on cmd line
vigra watershed workflow w/ classification: load .ilp file given on cmd line
DataSelection: Fixed a invalid viewer after last dataset is removed.
LayerViewerGui: Improved an assert message.
OpBatchIo: Added an output slot that provides the data from the exported file.
BatchIoGui: Added a viewer for the batch results.
Still needs the following improvements:
- Determine whether or not previously saved data can be shown instead of automatically assuming it's dirty.
(At the moment, if the export file already exists when the app is launched, it isn't shown.)
- Somehow decide how to display the image, instead of just using createStandardLayerFromSlot.
(e.g. pixelwise predictions should be shown as multiple alphamodulated layers, not a single RGB layer.)
- Add the raw data as a layer in the view
OpBatchIo: Always display the results from disk if they exist (and have the right shape).
OpBatchIo: disconnect internal op before cleaning it up.
BatchIoGui:
- Fixed a crash due to early slot access (before it was ready).
- During init, add any pre-existing images to the list (This couldn't happen before the image lane rewrite,
but is now necessary)
OpBatchIo: Comment.
Removed print statements.
PixelClassification Workflow: Now uses a custom gui for batch results viewing.
TODO: Actually display alpha-modulated layers instead of 1 RGB layer.
LabelingGui: When a new label is added, immediately call the "changed" callbacks so that any listeners are aware of the new label colors/name.
BatchIo: Show the raw data layer in the viewer, too.