Add extension method ReadGroups

[NoviProg]

This method make it easy to work groups. By use enumerator

like this:

var nos = new QuickFix.FIX42.NewOrderSingle();
// ...
foreach (var item in nos.ReadGroups<QuickFix.FIX42.NewOrderSingle.NoTradingSessionsGroup>(Tags.NoTradingSessions))
{
    // ...do stuff with noSidesGrp...
 }

[CLAassistant]

All committers have signed the CLA.

[gbirchmeier]

This is actually really cool, and I'm sorry I overlooked this when you first submitted it.

I might see if I can improve it a little bit tomorrow.

It would be really cool if we could make it not need the explicit counter-tag requirement, but there is currently no way in the code to get the tag from a group type. But I think I might be able to extend the code generator to give us something that could make that work... I'm going to keep thinking about it.