Separation of ShapefileDataStoreFactory and ShapefileDirectoryFactory

These two factories were written in a way that was hard to understand. With the commit "Separate ShapefileDataStoreFactory and ShapefileDirectoryFactory", I think I've made it more readable. ShapefileDirectoryFactory is still dependent on ShapefileDataStoreFactory to create ShapefileDataStore.

At the same time I removed the parameter file_type. This is given by wether the file is a directory or a file with extension shp. It might have some consequences that I haven't thought about.