Do not export `grid`

[pjaap]

1. grid is a grid and not an operator.
2. Pitfall during prototyping for new users if ExtendableFEM and ExtendableGrids are used.

As a breaking change, this would lead to a 2.0 release.

[chmerdon]

citation from #122:

And for the grid operator: This is only used for plotting, isn't it? Maybe we should change the interface of the plotting routines in ExFEM.

Yes, it is only used for plotting. How about using symbols instead, e.g., replacing plot([grid(u)], sol) by plot([(:grid, u)], sol)