Import improvement: added code and ui for extra parameters

I started changing the ui and code logic to allow for extra parameters to be specified during hdf5 import. I'm not sure the best way to implement this, but perhaps this code will be useful for you. So far, it allow the user to add a custom xrange and yrange to 2D datasets. There is an input box for "custom slicing", but I'm not quite sure how to implement that. I'll come back to that when next I get a chance.