Add zoom levels config model

[jacob720]

Adds a ZoomLevels config model to represent information in jCameraManZoomLevels.xml files. Both this model and DisplayConfig now inherit from a new class PerZoomLevel. This will be a breaking change as the import location for DisplayConfig and DisplayConfigData have moved from

from daq_config_server.models import DisplayConfig, DisplayConfigData

to

from daq_config_server.models.oav import DisplayConfig, DisplayConfigData