Modularize geogram (everybody does not need everything)

[BrunoLevy]

• make exploragram a separate repository
• make geogram-gfx a submodule: tryed to do that, but geogram-gfx has imgui as submodule, and when I remove geogram-gfx then add geogram-gfx as submodule, it complains about existing directory geogram-gfx (kept because there is geogram-gfx/third_party/imgui as submodule in it). Help needed here if a git wizard is available (I am terrified by git, so I will not do that on my own)

[FabienPean]

For point 2, I faced recently a similar problem, the solution would be to remove it from .gitmodules, git rm --cached on the folder and/or submodule folder, and then commit the removal.