Move `gc.collect` call to end of `Step.run`

[braingram]

Step.run calls gc.collect near the beginning of the function body:

stpipe/src/stpipe/step.py

Line 418 in 9952c36

gc.collect()

As it's likely that Step.process will result in allocations, it may be more useful to move this call to after Step.process.

[pllim]

#303