Update ZWO exposure time default parameters and make properties

[RemiSoummer]

The default values to bypass any correction to the exposure time in the setter or getter when interacting with the ZWO API should be:

exposure_time_step_size: 1

exposure_time_offset_correction: 0

exposure_time_base_step: 0

To Do:

• make these values properties of the service so that they can be more easily set to the defaults for calibration experiments.

fixes #434