Revert changes to retinotopy.yaml

[rly]

1. Revert the changes that were made in Refactor ImagingRetinotopy type to reduce redundancy #304 that reduced redundancy in the imaging retinotopy schema by way of introducing new neurodata types. These new types create a challenge for the APIs to read files created with an earlier version of ImagingRetinotopy that do not have the core spec cached. For the time being, until the API can be updated appropriately, these changes will be put on hold.

2. Apply some of the non-breaking changes from Refactor ImagingRetinotopy type to reduce redundancy #304 in specifying shape, and clarifying docs, dtype, and dims

3. Make sign_map and focal_depth_image datasets optional -- see Retinotopy improvements #407