add round when discretizing variable in quantics

[leigeng1997]

add a rounding function in the function "coord_to_id" to avoid incorrect conversions caused by numerical errors

[yurielnf]

Thanks for your proposal @leigeng1997 ! I have tried this branch locally and it doesn't pass the tests. However this can be fixed.
The problem I see is that this change has a non trivial effect on the convergence of tci. I need to observe in detail if the effect is systematically bad or not. Let's wait before merging ...