show all possible values in TelescopeParameters with (auto-generated) CaselessStrEnum traits

[kosack]

Please describe the use case that requires this feature.

create_class_enum_trait() can generate a configurable CaseLessStrEnum trait with all the possible values listed, e.g.

 --ImageProcessor.image_cleaner_type=<CaselessStrEnum>
     ImageCleaner to use.
     Choices: any of ['TailcutsImageCleaner', 'MARSImageCleaner', 'FACTImageCleaner'] (case-insensitive)
     Default: 'TailcutsImageCleaner'

This is very useful for letting the user know what options can be used.

However, this doesn't work when the trait is part of a TelescopeParameter, as in CameraCalibrator.extractor_type:

--CameraCalibrator.image_extractor_type=<telescopeparameter-item-1>...
    Name of the ImageExtractor subclass to be used.
    Default: [('type', '*', 'NeighborPeakWindowSum')]

Describe the solution you'd like

TelescopeParameters of CaselessStrEnums should show the possible Enum values in their help string.