Graph detach through JGF reader

[milroy]

As mentioned in issue #554 the JGF reader needs a new method that will unpack a JGF and remove the vertices and edges from the resource_graph. It appears that the best way to accomplish this is analogous to resource_reader_jgf_t::unpack: https://github.com/flux-framework/flux-sched/blob/f77983698a62aee4fc9dd0bc7ac2e392492d154d/resource/readers/resource_reader_jgf.cpp#L744 except that it must remove the vertices from planner (something like planner_remove), and update the pruning filter aggregates. Some of this is taken care of by https://github.com/flux-framework/flux-sched/blob/af4447ffd27400cc33a908c62201380a98075571/resource/traversers/dfu_impl_update.cpp#L529 To decrement the aggregate pruning filter JGF detach will need to call a new method that does the opposite of dfu_impl_t::accum_if: https://github.com/flux-framework/flux-sched/blob/af4447ffd27400cc33a908c62201380a98075571/resource/traversers/dfu_impl.cpp#L330