MASKNAME not in MPL-4 header

[havok2063]

Describe the bug
Initializing a Map fails in MPL-4 because MASKNAME is not in the header of MPL-4 data. So all the new maskbit and flag reorganization will fail for MPL-4 data.

To Reproduce

config.setRelease('MPL-4')
from marvin.tools.maps import Maps
maps = Maps('8485-1901')
ha = maps.emline_gflux_ha_6564

Expected behavior
It should initialize correctly.

Additional context
The correct maskbit names for all releases are in the datamodel, so we should consider changing the calls to header['MASKNAME'] to use the datamodel instead. Or alternatively, for MPL-4 headers, add the MASKNAME key when the header gets defined from the MPL-4 datamodel.

[albireox]

Let's combine it with #479 and add both qual_flag and pixmask_flag to the datamodel.

[havok2063]

Sounds good.