Better code design for boundary labeling in UW meshing

[gthyagi]

Currently, we can create a UW mesh by calling uw.meshing or by directly passing a DMPlex object or a Gmsh .h5 file to uw.discretisation.Mesh. In the first case, the internally defined boundaries class handles everything and is consistently used throughout the mesh creation process. However, in the second case, we need to manually pass the boundaries to uw.discretisation.Mesh. This can lead to issues if the names and values don't match between the DMPlex/.h5 file and the boundaries class provided. We need a better strategy to address this. Additionally, we must ensure that mesh.dm has boundary labels that are used for applying boundary conditions. This step should be mandatory before solving any set of equations on the mesh.