Modified the splitting rule for self tasks to be less conservative. Added proper documentation of the splitting conditions.

Also create functions to decide when to rebuild and when to split a task. Update the rebuild and split condition to correctly use kernel_gamma.

change sort task flags to be zero by default and only switch on those that are needed, avoids pre-emptively computing all the sorts for a cell since they may never be needed between two sort cleanups.

replace 'wait' with 'implicit' in scheduler_addtask, as we weren't using the former and could use the latter.

Removed the 'tight' property of the tasks. Only density pairs need to be tight so we can just use that.