Use temporary folder for tests

[chris-allan]

Slightly refactored version of the temporary folder test changes originally proposed in #70.

Test cases now use a temporary folder and subclasses are responsible for setting up the n5 implementation.

[chris-allan]

The original implementation in #70 had the temporary folder rule in the N5FSTest derived class. Since the AbstractN5Test base abstract class will be used by other filesystem based test harnesses I made the decision to place the JUnit resource management rule there. An argument could certainly be made that it should be the responsibility of the derived class to handle such resource management as in the case of object storage based test harnesses it will not be used.

Happy to go with either option.

Edit: The original abstract method style could also be restored with something to the effect of:

protected abstract N5Writer createN5Writer() throws IOException;

@Before
public void setUp() throws IOException {

  n5 = createN5Writer();
}

[chris-allan]

@axtimwalde: Was this what you were thinking?