Add readBulk to Array #99

Add a method that implements bulk reading of regions:

class OffsetAndShape {
   long[] offset;
   long[] shape;
}

class Array {
   public ucar.ma2.Array[] Array.readBulk(OffsetAndShape[] offsetsAndShapes) {}
}

The readBulk method can apply optimizations, such as reading in parallel or coalescing reads from the same shard, internally.