This PR adds:

• the flexibility to provide lon/lat coordinates from a text file (loaded with numpy loadtxt method) to build the graph;
• the flexibility to build 2D "flat" graphs with Voronoi in case SphericalVoronoi does not work well/is an overkill (e.g. in Switzerland, the effect of the spherical aspect of the Earth is minimal and SphericalVoronoi does not work so well for high resolution graphs, see previous PR/Issue). If flat is provided in the nodes attributes, the area weight is computed using Voronoi with specific qhull options "QJ Pp" preventing the empty region creation.

  nodes:
    area_weight:
      _target_: anemoi.graphs.nodes.attributes.AreaWeights # options: Area, Uniform
      norm: unit-max # options: l1, l2, unit-max, unit-sum, unit-std
      flat: true