Make TaskHubs only eligible for Task claims from compute services if they are associated with an 'active' AlchemicalNetwork

[dotsdl]

Expanding on #237, where we introduced the concept of AlchemicalNetwork state to allow users to set networks to states other than 'active' when they are no longer relevant, it is desirable for compute services to also not expend effort on non-'active' networks. Users generally don't expect networks that are not 'active' to still receive valuable compute attention.

To achieve this, we propose making Neo4jStore.query_taskhubs take a network_state keyword argument to filter on network state, by default giving only TaskHubs associated with 'active' networks.