Explicitly del model in ModelLibrary.map_function

[braingram]

In the following loop:

stpipe/src/stpipe/library.py

Lines 739 to 745 in 9952c36

	for index, model in enumerate(self):
	try:
	yield function(model, index)
	finally:
	# this is in a finally to allow cleanup if the generator is
	# deleted after it finishes (when it's not fully consumed)
	self.shelve(model, index, modify)

The lack of an explicit del model after the shelve keeps it in scope until it's overwritten by the enumerate iterator which only overwrites it after the next model is loaded. This results in 2 models in memory when only 1 is needed.