Add Pellet I/O and comparison utilities

[mgyoo86]

Prerequiste

Dependencies: This PR builds on top of perf/simple_trace branch

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Summary

Adds HDF5 serialization support and comprehensive equality operators for the Pellet type. Refactors type system for better maintainability.

Changes

Type System Refactoring

• Simplify Pellet from 7 type parameters to 1 (FT<:AbstractFloat)
• Extract type definitions to new src/types.jl module

HDF5 I/O Support for Pellet struct

• pellet2hdf(pellet, filename) - serialize Pellet to HDF5
• hdf2pellet(filename) - deserialize from HDF5 to Pellet

Equality Operators

• Base.isequal
• Base.:(==)
• Base.isapprox
• Base.diff

New Dependencies

• HDF5.jl - for file I/O
• StaticArrays.jl - for velocity_vector field

Usage Example

import PAM

# Run simulation
pellet = PAM.run_PAM(dd; t_start=0.0, t_finish=0.1, time_step=0.01, ...)

# Save to file
PAM.pellet2hdf(pellet, "pellet_output.h5")

# Load from file
loaded = PAM.hdf2pellet("pellet_output.h5")

# Compare results
loaded == pellet          # Exact equality
loaded ≈ pellet           # Approximate equality
diff(loaded, pellet)      # Show differences

Test Coverage

• All new functionality is tested ✅