Add documentation to all processing algorithms

[nmaarnio]

_short_help_string should be extended for all processing algorithms. It should have the general description in docstring of the corresponding tool in EIS Toolkit, but can be extended/modified to better fit this context.

pca_vector.py and pca_raster.pycan be used as examples.

The documentation should be added in batches, for example category at a time.

• [x] Raster processing
• [x] Vector processing
• [x] Transformations
• [x] Exploratory analyses
• [x] Prediction
• [ ] Utility
• [ ] Evaluation
• [x] (Filters)