Guidance on cd_values and cd_nelems

Is the goal of cd_values and cd_nelems to help **decode** or **reencode** with the exact same parameters?

I'm thinking of:

* load data
* transform data
* Store transformed data

and perhaps you want to store in the "exact same way as b4".

> You are welcome! Remember to contact again once the `cd_nelems` and `cd_values[]` of your filter plugin are defined. This info is the essential part of a filter plugin definition.

_Originally posted by @ajelenak in https://github.com/HDFGroup/hdf5_plugins/issues/215#issuecomment-3774653022_

If it is just about "reading" data, i think perhaps there may not be much needed?????

Can these be variable lengthed???