Add pixel_replace step to MOS Master Background Processing

[stscijgbot-jp]

Issue JP-3748 was created on JIRA by Christian Hayes:

The MOS master background step in spec2 calls processes background slits through most spec2 steps in order to calculate a combined 1D background spectrum, but these steps do not include the pixel_replace step, which might be desired as an optional step for filling missing data in background slits.  The most logical place to include this may be in master_background.nirspec_utils.create_background_from_multislit before the resample spec step is called (to mirror where the pipeline is currently calling pixel_replace).
 
As with the ResampleSpecStep and Extract1dStep in https://jira.stsci.edu/browse/JP-3598, it might be useful to also allow users to modify the parameters of pixel_replace separately for the background and science data.