Spark Kernel adaptation of SparkR

Presently, the following APIs are made private in SparkR that are used by the kernel to provide a form of communicate suitable for use as an interpreter:

1. SparkR only has an init() method that connects to the backend service for R and creates a SparkContext instance. That I am aware, there is no other way to currently use SparkR. Because of this, a new method labelled sparkR.connect() is used that retrieves the existing port under the environment variable EXISTING_SPARKR_BACKEND_PORT. This method is located in sparkR.R and is exported via the following:

    export("sparkR.connect")
   

2. SparkR low-level methods to communicate with the backend were marked private, but are used to communicate with our own bridge. If you need to use these invoke them with

    SparkR:::isInstanceOf
    SparkR:::callJMethod
    SparkR:::callJStatic
    SparkR:::newJObject
    SparkR:::removeJObject
    SparkR:::isRemoveMethod
    SparkR:::invokeJava
   

3. org.apache.spark.api.r.RBackend is marked as limited access to the package scope of org.apache.spark.api.r

   - To circumvent, use a reflective wrapping under 
     `org.apache.toree.kernel.interpreter.r.ReflectiveRBackend`