Audit and update SdssShape

The SdssShape contains a number of poorly-motivated numerical constants ("magic numbers", e.g. here) and inconsistent sanity checks (compare here and here).

Please audit the code for these issues. In particular:

• Check for numerical constants and ensure their values are appropriate and properly justified.
• Ensure that checks on the shape determinant are consistent, including both those listed above and those added in DM-3935.