Parallel: clarify javadoc

Clarify that the op implementing the Parallel interface will spawn
multiple threads -- *not* that the Parallel op is intended to be *used*
simultaneously from multiple threads. (That would be a Threadable op.)
Rename threading package to chunker

The naming scheme we settled on is to use imagej.ops.<foo>,
where <foo> matches the name of the ops being implemented.
Simplify chunker op class names

The ChunkExecutor has been renamed to Chunker, to match the naming
convention that op interfaces match the name of their operation.

The ChunkExecutable has been renamed to Chunk, since it is shorter and
matches the naming of Chunker.

A Chunker executes Chunks of code! So nice! ;-)
Add Threadable interface

See: https://github.com/imagej/imagej-ops/issues/5
Parallel: flesh out the javadoc
Chunk: improve javadoc
Function: tweak javadoc
Function: implement the Threadable interface
Chunker: improve javadoc

Now it is consistent with the other base interfaces for ops.
Use Chunker.NAME constant where appropriate
ParallelMappers: Using thread-safe functions now
Merge branch 'master' into threadable
AbstractJoinFunctionAndFunction: Declare fields as private
AbstractJoinFunctionAndFunction: Added getter for BufferFactory
DefaultJoinInplaceAndInplace: Rename parameter
DefaultJoinInplaceAndInplace: formatting
JoinFunctions: Adding JavaDoc
Loop: Loop requires number of iterations.
AbstractLoopFunction: Declared fields as protected

* Added getter/setters to access fields
AbstractLoopInplace: Declared fields as private

* and added getter/setter