Implement GROUP and layer number system

[tngreene]

If using _PARAMs, we're using the GROUP system. If not, each sub collection just gives a layer number.

In either case the representation is the same - the 2nd level collections are the the form of "

• Unit for good cases
  • GROUP and layer numbers given properly
  • layer number comes from name
  • collection with just "1" or whatever for a name still work
  • empty sub collection is ignored
  • All trees under the same collection share the same layer number, even if linking was used
• Unit test for bad cases
  • bad names "1badname", "no number", "1.234 float used instead", "-1 negative number given", etc
  • duplicate layer numbers made - no collections "2 layer numbers" and "2 another collection"
• Code for GROUP system
• Code for regular layer system

[tngreene]

Unit test is part way along, wouldn't take much to finish it