Adding option for mesh refinement

[jon-proximafusion]

Mesh refinement might be useful in some cases

Here is an example but it might be best if the user does the mesh manually in this the case that they need mesh refinement

optional mesh refinement example
refine_distance = 50  # Distance within which to refine the mesh
refine_mesh_size = 2  # Mesh size within the refine distance
# Define a distance field for mesh refinement
gmsh.model.mesh.field.add("Distance", 1)
gmsh.model.mesh.field.setNumbers(1, "NodesList", [1,2,3,4])
# Define a threshold field to refine the mesh within a certain distance
gmsh.model.mesh.field.add("Threshold", 2)
gmsh.model.mesh.field.setNumber(2, "InField", 1)
gmsh.model.mesh.field.setNumber(2, "SizeMin", refine_mesh_size)
gmsh.model.mesh.field.setNumber(2, "SizeMax", 40)
gmsh.model.mesh.field.setNumber(2, "DistMin", 0)
gmsh.model.mesh.field.setNumber(2, "DistMax", refine_distance)