new method for consumers to ignore offers

added to allow minerals, builders and other consumers to notify managers
that they cannot receive new offers at the moment (but may allow it later)
such as when a mineral patch alreday received a worker and thus cannot
accept a new one while it's busy being gathered
implemented new canReceiveOffer() method in consumers
deleted obsolete agent (replaced by MakeStructure)
added call to consumer to check if it can receive offers