Change slicing logic to avoid storing global data

[Georacer]

#47 created another property to store an index of the LogMsgGroups, so that it can iterate over them during slicing.

It would be better to avoid property inflation and use 'isa()' to parse LogMsgGroup properties upon request.

[Georacer]

While we're at it, might as well upgrade the copy function, as per the comments in that PR.