delete_vertices_idx() and induced_subgraph_map() have inconsistent names

[szhorvat]

igraph_delete_vertices_idx() and igraph_induced_subgraph_map() perform comparable operations: they subset the graph's vertices, and output a mapping and inverse mapping between the old and new vertex IDs. However, their names are confusingly inconsistent.

I propose renaming igraph_delete_vertices_idx() to igraph_delete_vertices_map() on 0.11 only.

Note that on develop, the behaviour of these functions has already been changed (now the mapping uses 0-based indexing). So let's not do the renaming/deprecation on master.

@vtraag and @ntamas, any objections?

[ntamas]

Implemented in the commit above; CI failures are unrelated (related to a merge from master back into develop).