Refactor Parallel_Grid::init into serial/parallel subroutines

[MrLi000001]

Motivation The original Parallel_Grid::init used an in-function #ifndef __MPI ... return; guard, which effectively split the function into two halves. Readers had to infer that "the serial build stops here while the parallel build continues", violating clear control-flow and increasing maintenance burden.

Changes

Split the original init into two single-responsibility functions:
init_serial(...): assigns dimension members, validates nczp < 0, asserts sizes, and computes ncxy/ncxyz (shared by both builds).
init_parallel(nprocgroup): partitions processes across Pools by KPAR, allocates numz/startz/whichpro/whichpro_loc, and calls z_distribution().
The new init(...) orchestrates both: calls init_serial always, then init_parallel under #ifdef __MPI.
Removed the old init implementation and a now-stale comment, eliminating the mid-function #ifndef __MPI early return.
Added init_serial / init_parallel private declarations to the header.
Result

Logically equivalent; behavior is unchanged.
Serial vs. parallel code paths are now clearly separated, improving readability and maintainability.
Testing

Serial build (-DENABLE_MPI=OFF) passes
Parallel build (-DENABLE_MPI=ON) passes

[github-actions]

Agent Governance Check

Severity	Rule	Location	Reason	Suggested action	Exception
warning	PR metadata completeness	pull_request.body	missing sections: Linked Issue, Unit Tests and/or Case Tests for my changes, What's changed?	Fill the PR template with issue linkage, test evidence, and a concise change summary.	allowed
warning	Test evidence review	pull_request.body	Source code changed without test path changes or PR test evidence.	Add focused tests, update a relevant case, or document why tests are not required.	allowed
warning	Documentation sync review	pull_request.body	Source changes have no docs change or explicit no-docs-needed statement.	Add documentation updates for behavior/interface changes, or state why documentation is not required.	allowed