Lowest order virtual Element Method on polygons for the 2D Poisson problem based on the grid manager ExtendableGrids and the sparse matrix manager ExtendableSparse.

The main assembly loop is inspired by:
Sutton, O.J. The virtual element method in 50 lines of MATLAB. Numer Algor 75, 1141–1159 (2017). https://doi.org/10.1007/s11075-016-0235-3

There is an example that can be run in the julia REPL by:

julia> using PyPlot
julia> include("examples/Example200_PoissonVEM.jl")
julia> Example200_PoissonVEM.main(; Plotter = PyPlot)