Apply spectral smoothing for MRS master background subtraction

[stscijgbot-jp]

Issue JP-3758 was created on JIRA by David Law:

MRS master background currently averages over the spatial area within each wavelength plane, but this still introduces significant noise into the data as the background signal is often dominated by detector noise.  Better results may be obtained by enforcing a degree of spectral smoothness in the master background estimated from dedicated sky observations.

Assignment to MIRI team to study.