[Java]: Add "sbe" to front of blockLength and headerSize in group to avoid name conflicts.

Author: mjpt777

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

@@ -259,14 +259,14 @@ private void generateGroupClassHeader(final StringBuilder sb,
  ));
 
  sb.append(String.format(
- indent + " public static int headerSize()\n" +
+ indent + " public static int sbeHeaderSize()\n" +
  indent + " {\n" +
  indent + " return HEADER_SIZE;\n" +
  indent + " }\n\n"
  ));
 
  sb.append(String.format(
- indent + " public static int blockLength()\n" +
+ indent + " public static int sbeBlockLength()\n" +
  indent + " {\n" +
  indent + " return %d;\n" +
  indent + " }\n\n",