add 3 methods to `Metadata` class: `containsCategoryDiff()`, `getMaterialLocatorLeft()`, `getMaterialLocatorRight()`

[kazurayam]

At https://github.com/kazurayam/inspectus/blob/0.5.3/core/src/test/java/com/kazurayam/inspectus/fn/FnChronosDiffTest.java , I want to write a code as follows:

        JobTimestamp jtLatest = store.findLatestJobTimestamp(jobName);
        MaterialList ml = store.select(jobName, jtLatest);
        ml.forEach ( material -> {
            assertTrue(material.getMetadata().containsCategoryDiff());
            assertEquals(jt1, material.getMetadata().getMaterialLocatorLeft().getJobTimestamp());
            assertEquals(jt3, material.getMetadata().getMaterialLocatorRight().getJobTimestamp());
        });

So, I need to extend the com.kazurayam.materialstore.core.filesystem.metadata.Metadata class. It should implement:

• boolean containsCategoryDiff()
• MaterialLocator getMaterialLocatorLeft()
• MaterialLocator getMaterialLocatorRight()

[kazurayam]

done at 0.12.3-SNAPSHOT