[Java]: Provided the ability to access group block length. Enhancement aeron-io#92.

Author: mjpt777

main/java/uk/co/real_logic/sbe/generation/java/JavaGenerator.java

@@ -259,6 +259,18 @@ private void generateGroupClassHeader(final StringBuilder sb,
  ));
 
  sb.append(String.format(
+ indent + " public static int blockLength()\n" +
+ indent + " {\n" +
+ indent + " return %d;\n" +
+ indent + " }\n\n",
+ blockLength
+ ));
+
+ sb.append(String.format(
+ indent + " public int actingBlockLength()\n" +
+ indent + " {\n" +
+ indent + " return blockLength;\n" +
+ indent + " }\n\n" +
  indent + " public int count()\n" +
  indent + " {\n" +
  indent + " return count;\n" +