[meshlib] Meshing features for shapefiles and allow reading of more mesh formats

Changes to allows the meshing via GMSH or Triangle of shapefiles:

• read shapefile with mesh class to construct a mesh, with default or individual parameters to decide meshing algorithm or simplification
• read shapefile with independent meshlib function
• some private functions
  • to create a mesh from points and cells if all celltypes are homogeneous
  • to create points and cells for a triangular mesh
  • prepare a shapefile for meshing as a GeoDataFrame

There is also an corresponding cli for the main features.

Also Mesh class can now read all formats compatible with meshio.

An example of a shapefile meshed with triangle: