subset, transition with hfun

[felicio93]

With the latest PR #174, the utils function merge_overlapping_meshes takes the argument: hfun_mesh.

This is a msh_t object that will be used as the transition between the low priority (e.g. floodplain, coarse mesh) and high priority (e.g. river, hurricane track) mesh.

We need to update the subset_n_combine to pass its mesh_buf_apprx to merge_overlapping_meshes as hfun_mesh