feat(graph): association is not causation

Adds co_occurs_with, temporally_precedes, possibly_caused_by and
asserted_by to the relation vocabulary with Causal and Hypothetical
predicates. Entities mentioned together now co-occur rather than being
'related'; association hypotheses record possibly_caused_by and their
basis (temporal association only); caused_by is reserved for explicit
operator statements and nothing automatic writes it. Documented in
docs/graph.md and checked end to end.

pkg/concepts/relations_test.go

@@ -0,0 +1,17 @@
+package concepts
+
+import "testing"
+
+func TestRelationSemantics(t *testing.T) {
+	for _, r := range []Relation{CoOccursWith, PossiblyCausedBy, TemporallyPrecedes, AssertedBy, CausedBy} {
+		if !r.Valid() {
+			t.Fatalf("%s should be valid", r)
+		}
+	}
+	if !CausedBy.Causal() || PossiblyCausedBy.Causal() || CoOccursWith.Causal() {
+		t.Fatal("only caused_by asserts a cause")
+	}
+	if !PossiblyCausedBy.Hypothetical() || CausedBy.Hypothetical() {
+		t.Fatal("possibly_caused_by is the hypothetical form")
+	}
+}