add fast→thermal particle source

[nanshi1177]

Implement fast→thermal particle source handling in fast_particles_profiles!

• Extends fast_particles_profiles! to accumulate thermalized particle sources generated from fast ions (e.g., NBI, fusion)
• Adds creation of corresponding :fast_thermalization entry in core_sources
• Initially introduced to provide a proper source term for evolving thermal He ash; later generalized to handle all fast ion species consistently
• Requires a matching identifier addition (:fast_thermalization, index 307) in IMASdd.jl.

[nanshi1177]

The previous approach is no longer valid. I will open a new PR with a revised implementation.