Use dask checkpoint function in dask executor

Dask recently released some advanced graph manipulation tools, which includes a checkpoint function.

We could use this instead of our low-level hack to make a checkpoint here:

Lines 51 to 53 in 6cc0f26

    
           def merge_all(*args): 
        
               # this function is dependent on its arguments but doesn't actually do anything 
        
               return None

	def merge_all(*args):
	# this function is dependent on its arguments but doesn't actually do anything
	return None