Add clean!(::Triangulation) and clean!(::VoronoiTessellation) (with better names)

[DanielVandH]

These methods would redefine the connectivities and the point lists so that only points in the triangulation/tessellation are included in the connectivities. It would also remove any empty features like in https://github.com/JuliaGeometry/DelaunayTriangulation.jl/blob/d3bdb2e9d5518bf00748e26a1d28b745289ed082/src/algorithms/triangulation/basic_operations/clear_empty_features.jl

cc: @juliohm

[DanielVandH]

Might not be able to do this in-place since not every users point set will be mutable. Maybe just make two methods repair and repair!, where repair will recreate the triangulation and make a new point set as a vector of tuples

[DanielVandH]

Repair is a bad name incase I want to implement e.g. shewchuks repair algorithm for perturbed data