Increase visibility for ADAMContext.sc and/or getFs... methods

[heuermh]

The Hadoop configuration from the SparkContext associated with an ADAMContext needs to be accessible for subclasses to access the FileSystem

[INFO] -------------------------------------------------------------
[ERROR] COMPILATION ERROR : 
[INFO] -------------------------------------------------------------
[ERROR] .../org/dishevelled/bio/adam/BiojavaAdamContext.java:[136,52]
  sc has private access in org.bdgenomics.adam.rdd.ADAMContext

Alternately, we could increase the visibility of the getFs... methods from private[rdd] to protected.

[fnothaft]

Ah, I hadn't thought of the Java access patterns here. I think we should just make the sc field of ADAMContext public again.

[heuermh]

The workaround is not so bad, I wrap sc in the constructor into a JavaSparkContext field

public class MyAdamContext extends ADAMContext {
  private final transient JavaSparkContext javaSparkContext;

  public MyAdamContext(final SparkContext sc) {
    super(sc);
    javaSparkContext = new JavaSparkContext(sc);
  }

  //...

  FileSystem fileSystem = path.getFileSystem(javaSparkContext.hadoopConfiguration());

https://github.com/heuermh/dishevelled-bio/blob/master/adam/src/main/java/org/dishevelled/bio/adam/BiojavaAdamContext.java#L133

What do you think about the getFs... methods?