Fixed an issue on getProductComponent

Fixed an issue on getProductComponent for SpotDimapProductReader: when
the thunb file didn't exist, the path file was not added also. Now it
is!
ZipDir is now a private member

ZipDir is now a private member of SpotViewProductReader, since it is
needed to take the component files and the method getProductComponents
is is implemented
tests for SimpleProductReader and volumeproductreader were separated

tests for SimpleProductReader and volumeproductreader were separated
test class for SpotViewProductReader

test class for SpotViewProductReader