[MPI] Fix int to SimplexId

[eve-le-guillou]

Hi all,

In edgeToPosition of both Implicit and Periodic Triangulations, an intermediary variable (e) was set and was an integer. This worked only up until a certain number of edges.
For a grid of $2048$ x $2048$ x $1024$, a problem would arise, as the computed number needed a long int type to be expressed and was expressed as an int. The result was negative integer positions (impossible in our grids).

Setting theint as a ttk::SimplexId solves the problem.

For triangleToPosition, the variable (here t) is already a ttk::SimplexId.

Thanks for any feedback.

[julien-tierny]

looks good to me, thanks a lot!