Change naming of node `key` to `id`

Change naming convention so that we use only id instead of key. For that, the following changes are needed:

• build_simplex_index_key_maps -> build_simplex_index_id_maps
• simplex_index_to_key -> simplex_index_to_id
• node_index_to_key -> node_index_to_id

key refers to the key in the unordered_maps of mesh nodes and simplices (which is different to the indices of such mesh nodes (and associated repatoms) and simplices in vectors such as qc_mesh.repatoms.locations.