Ensure generation_time is 1 when time_units are generations

Closes popsim-consortium#125.

examples/browning_america.yaml

@@ -2,10 +2,6 @@ description: The Browning et al. (2011) model of admixture in the Americas.
 doi:
   - https://doi.org/10.1371/journal.pgen.1007385
 time_units: generations
-# TODO the parser currently raises an error if we set this value. See the
-# note in the code for details.
-# generation_time: 25
-
 demes:
 - name: ancestral
   description: Equilibrium/root population

reference_implementation/parser.py

@@ -566,6 +566,10 @@ def validate(self):
                     "Must specify Graph.generation_time if time_units is not "
                     "'generations'"
                 )
+        if self.time_units == "generations" and self.generation_time != 1:
+            raise ValueError(
+                "If time_units are in generations, generation_time must be 1"
+            )
         for deme in self.demes.values():
             deme.validate()
         for pulse in self.pulses:

reference_implementation/tests.py

@@ -101,12 +101,17 @@ def test_time_units(self):
     def test_generation_time_generations(self):
         data = minimal_graph()
         data["time_units"] = "generations"
-        data["generation_time"] = 12
-        parser.parse(data)
         data["generation_time"] = 1
         graph = parser.parse(data)
         assert graph.generation_time == 1
 
+    def test_bad_generation_time(self):
+        data = minimal_graph()
+        data["time_units"] = "generations"
+        data["generation_time"] = 12
+        with pytest.raises(ValueError):
+            parser.parse(data)
+
     def test_generation_time(self):
         data = minimal_graph()
         data["time_units"] = "years"