rebase and improve PR #741 - ReadGroups

[gbirchmeier]

This was a cool submission, @NoviProg! I cleaned it up a little bit, and also promoted it to being a full-fledged FieldMap member instead of an extension.

( closes #741 )

Summary:

It adds a FieldMap method ReadGroups to allow you to iterate through all the elements of a repeating group.

Here is a simple example to iterate through all the lines of a News message:

    foreach (var item in msg.ReadGroups<News.LinesOfTextGroup>(Tags.NoLinesOfText)) {
        System.Console.WriteLine(item.Text.Value);
    }

I wish there was a way to eliminate either the template param or the tag param, but I couldn't crack it.

I will add an example to the web docs also.

[gbirchmeier]

@NoviProg let me know if you want to have your name on our Contributors page (and if yes, tell me what your name is!)

[NoviProg]

Thank you very much for your feedback. I'm glad you accepted my pull request. That's enough for me.