Add optional multiphase support (Shan-Chen & phase-field) with I/O, initialization and utilities

[gaoak]

Motivation

• Introduce optional multiphase physics to the solver so users can run single-phase, Shan‑Chen pseudo-potential, or phase‑field simulations and output phase/density fields per block.
• Add robust parsing for an optional MultiPhase block in input files and include multiphase parameters in parameter dumps for reproducibility.
• Fix a couple of indexing/validation issues in body placement and probe output selection.

Description

• Added MPModelNone/MPModelShanChen/MPModelPhaseField constants in ConstParams and new multiphase fields to FlowCondType in FlowCondition with defaults, and implemented parsing of an optional MultiPhase keyword using a new found_keyword_optional utility.
• Implemented multiphase data structures and algorithms in FluidDomain: allocate phi, phi_new, and chemPot for phase‑field, initialize phi and block densities (or mixed density for Shan‑Chen), update densities in collision, and added per-block routines apply_shan_chen_force, advance_phase_field, apply_phase_field_force, pseudopotential, and helpers for neighbours/gradients/laplacian.
• Enhanced I/O: write_flow_ now writes a Phase*****_b*** file when multiphase is enabled (density and phase indicator for phase‑field), adjusted output indexing/filename handling, and extended write_continue_/read_continue_ to save/load phase data when present.
• Added parameter printing of multiphase settings in write_parameter_check_file and a new optional keyword search routine found_keyword_optional in Util.f90.
• Minor fixes: corrected multi-dimensional placement indexing in Solidbody.f90 (lineY/lineZ computation) and added bounds checking for flow%inWhichBlock in main.f90 to avoid invalid probe access.

Testing

• Project compiled successfully with gfortran -O3 after the changes and no compile errors were reported.
• Executed short smoke simulations: a single‑phase baseline run completed and produced unchanged DatFlow outputs, and small multiphase smoke cases for both MPModelShanChen and MPModelPhaseField produced Phase..._b... files with density and phase fields as expected.
• Reading/writing of continue files including phase data was exercised in an automated short restart test and succeeded.

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Codex Task