Add rechunking option for the `EchoData` object

[valentina-s]

The recent PR #939 provides the option to have a default rechunking in to_zarr. In some cases, though, maybe the user wants to rechunk all data variables in the object in a very specific chunk dimensions good for their follow up analysis. It would be nice to have something like echodata.(re)chunk(...)

A few considerations brought up by @lsetiawan and @b-reyes :

• checking if the objects are already chunked
• chunk only those that are not chunked, or have some default option to overwrite existing chunking
• option to specify which variables to (re)chunk