src/gsp_nico: remove it

[jeromeetienne]

• obsolete code
• remove it, it is pollution to keep very obsolete code

Prompt

• remove code in ./src/gsp_nico and ./examples_nico
• and all its related code

[jeromeetienne]

Addressed in 7f6045c on branch mesh_ng — removed src/gsp_nico/ and examples_nico/, dropped ./src/gsp_nico/ from the Makefile pyright target, and removed sandbox mentions from the whitepaper docs and prompt inputs.