Dataset load speed up

[dale-wahl]

Really just a refactoring of DataSet.children attribute to a DataSet.get_children() method. It is particularly helpful with datasets that have many children with their own children since loading any dataset would automatically load all its children (and thus all of their children). Now they are only loaded if needed. (Biggest effect I see is in the results page speed on my local 4CAT.)

This also ensures the children are current when running certain methods (e.g. copy_ownership_from) that would update those children (previously they may not have been updated if they were created after a DataSet object was instantiated).

It does remove the is_finished filter that was previously applied to the DataSet.children attribute , but, so far as I can tell, that was never necessary where the attribute is used.

Also includes a debug time_this method which is only active when in debug mode. Could be applied elsewhere.