Respect map item atlas scaling mode

[rldhont]

The plugin forced the map item scaling mode to predefined scales with hardcoded scales.
from https://github.com/3liz/qgis-atlasprint/blob/master/filters/atlasprintFilter.py#L241 to https://github.com/3liz/qgis-atlasprint/blob/master/filters/atlasprintFilter.py#L243

        atlas_map.setAtlasScalingMode(QgsLayoutItemMap.Predefined)

        layout.reportContext().setPredefinedScales(predefined_scales)

It will be great to respect the defined map item scaling mode https://github.com/qgis/QGIS/blob/2c69b24ffe903a0d0d7fac59bbfaee02d3ceaa75/src/core/layout/qgslayoutitemmap.h#L48:

• Fixed
• Predefined
• Auto