Apply flatfield in mmirs_qrp

[astrochun]

mmirs_qrp.py has not been designed to use flat fields. Add a 'flat' keyword option of either integer values or filenames. Then the code will combine the flats and flatfield images before A-B image differencing.

[astrochun]

Begun to implement flat fielding in 4e39e6b but having some unusual issues with the final stacked products. Not sure the cause; will debug further later.