Add new MemoryStatus interface to Theta.

[leerho]

While going through the Java library, there are three public methods that had to do with the status of the internal Memory object used for a backing store: hasMemory(), isDirect(), and isSameResource(). These methods are only relevant to the sketch classes that need to access and retain the sketch as a Memory object, on or off-heap. But these three methods were inconsistently applied even to the classes that could use them. Some classes had only one or two of these methods implemented instead of all three.

Also because these methods in the root classes were implemented as abstract, it required all child dependents that did not need to access and retain the sketch as a Memory object, to implement these three methods as dummy methods that always returned false. This really clutters up the APIs of these classes and is "crazy-making" when trying to debug memory-related issues.

I thought it would be a good idea to clean this up. I have implemented a very simple interface, MemoryStatus, in o.a.ds.common with these three methods implemented as default methods that always return false. This way the only classes where these methods are relevant need to override these methods. And all the other classes don't need to implement dummy methods at all.

These methods are widely used across the library, but for this PR I have only implemented the necessary changes to the Theta tree to keep the PR smaller. Nonetheless there are 14 classes that were impacted, (an additional two classes had only spelling corrections). A summary of the changes for each file is included with the Files Changed View.

[leerho]

This is the new MemoryStatus interface that can be used eventually for the whole library. So far, this has only been applied to the Theta tree.

[leerho]

Memory not used as a backing store

• Removed import to Memory
• Removed isSameResource

[leerho]

isSameResource(Memory that) was missing.

[leerho]

This is a Base Interface

• added extends MemoryStatus & import - default is OK here
• removed hasMemory(), isDirect(), did not have isSameResource()

[leerho]

Memory used as backing store

• Updated hasMemory(), isDirect(), isSameResource() to check if mem != null

[leerho]

Memory used as backing store

• Updated hasMemory(), isDirect(), isSameResource() to check if mem != null

[leerho]

Memory not used as a backing store

• Removed hasMemory(), isDirect(), (did not have isSameResource())

[leerho]

Memory not used as a backing store

• Removed hasMemory(), isDirect(), did not have isSameResource()
• Note: need to remove getMemory()

[leerho]

Memory not used as a backing store

• Removed hasMemory(), isDirect(), did not have isSameResource()

[leerho]

Memory used as backing store

• Added hasMemory(), isDirect(), updated isSameResource()

[leerho]

Base abstract class

• Added implements MemoryStatus and import MemoryStatus
• Removed abstract isSameResource()

[leerho]

Memory not used as a backing store

• Removed hasMemory(), isDirect(), did not have isSameResource()

[leerho]

Base abstract class

• Added implements MemoryStatus and import MemoryStatus
• Removed hasMemory(), isDirect(), isSameResource()

[leerho]

Memory used as backing store via gadget

• Added hasMemory(), isDirect(), updated isSameResource()

[leerho]

Removed extra space