Store field transformation information in the CRDCH model

[gaurav]

Current work on the GDC- and PDC-to-CRDCH transformation code in the Example Data repo suggests that many transformations are relatively straightforward, such as one field renamed as another field. Some of this information could potentially be stored in the CRDCH Model itself, so that automated transformation could use the model information to transform a lot of data without any additional instructions.

Note that this is closely tied to cancerDHC/example-data#30, which is the same problem viewed the other way: how to record what transformations were made, rather than those that need to be made.

I've currently marked this as Future Goals, since I don't think this is an immediate priority, but it'd be nice for LinkML developers to think about this.