Aggregate all implicit activities and active forms of Agents.
Iterate over self.statements and collect the implied activities
and active forms of Agents that appear in the Statements.
Note that using this function to collect implied Agent activities can
be risky. Assume, for instance, that a Statement from a reading
system states that EGF bound to EGFR phosphorylates ERK. This would
be interpreted as implicit evidence for the EGFR-bound form of EGF
to have ‘kinase’ activity, which is clearly incorrect.
In contrast the alternative pair of this function:
gather_explicit_activities collects only explicitly stated activities.