Remove hardcoded values for zk_rows and num_chunks in Pickles code

[querolita]

This is part of the investigation of the chunking bug in Pickles and o1js (RFCs here and here).

Partially addresses o1-labs/o1js#1806

Explain your changes:

• Substitute hardcoded values of 3 for zk_rows for a variables called zk_rows_by_default, and likewise for 1 and num_chunks.
• This is to facilitate debugging and reduce error proneness.

Explain how you tested your changes:

• It only contains cosmetic changes.
• There must still be a few places in our codebase, because the chunking tests in o1js still read 3 always.

Checklist:

• Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• Modified the current draft of release notes with details on what is completed or incomplete within this project
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them

• Closes #0000

[volhovm]

LGTM

[querolita]

!ci-build-me

[querolita]

!ci-build-me