Add Const Definitions for Public Group Length Codes to Tag

[jfphilbin]

For each group in tag/tag.dart add a definition like the following:

static const Tag kGroupLength0008
//(0008,0000)
= const Tag.public("GroupLength0008", 0x00080000, "Group 0008 Length", VR.kUL, VM.k1, true);

Remove GroupLengthTag class from tag.dart