Preserve vertex labels after deleting a vertex.

[rachithaiyappa]

https://github.com/JuliaGraphs/LightGraphs.jl/blob/ddaf3d37aed00b2b22705376bd79fba2c4bb6363/src/SimpleGraphs/SimpleGraphs.jl#L124-L153

The documentation says node labels (indices) are lost when a node is removed from the graph. Perhaps it might be helpful to have an option to preserve it?

[etiennedeg]

The internal structure of the SimpleGraph maintains the vertices to a compact range starting from 1. This is part of the API contract of a SimpleGraph, and some algorithms may use this property. So we can't change it as we would not know which algorithms would break.