some little improvements

Most of my work in this last days has been solving minor bugs and adding small improvements to the SEXTANTE GUI. Here is a quick review of some of them. Not all of them have been finished and tested yet.

-Optional outputs. There is a new output channel (NullOutputChannel) that basically does nothing with the result of an algorithm. It can be selected from the output channel selection dialog, and doing so causes SEXTANTE not to create the corresponding output layer. This was, you can have optional outputs, but there is no need to set them as optional in the source code of each algorithm o take that into account somehow.

-GRASS algorithms in modeler. GRASS algorithms work now in the modeler, and I have already implemented a mechanism for dealing with the major issue: layers with more than one geometry type. When you execute a GRASS algorithm now, in case it generates vector layers with muliple geometry types, you have to select the type of geometry you want to use (the other ones are discarded). The selection is already implemented, but the GrassAlgorithm class is not using it yet. I will code that in a month or so, when I go to Salzburg to atend the AGIT conference, since we plan to have a code sprint there with other developers who know more than me about GRASS...

-Definition of analysis extent using stored points. Point stored using the catch coordinates tool can now be used to manually define the extent of the analysis region.