Work around sphinx cross-ref bug.

The error is:
```
stdpopsim/species.py:docstring of stdpopsim.Species::more than one target found for cross-reference 'id': stdpopsim.engines._MsprimeEngine.id, stdpopsim.slim_engine._SLiMEngine.id
```

See currently open sphinx issue for workaround.
sphinx-doc/sphinx#2549

Author: grahamgower

stdpopsim/engines.py

@@ -47,10 +47,10 @@ class Engine(object):
     be set, and the :func:`simulate()` and :func:`get_version()` methods must
     be implemented. See msprime example in ``engines.py``.
 
-    :ivar id: The unique identifier for the simulation engine.
-    :vartype id: str
-    :ivar description: A short description of this engine.
-    :vartype description: str
+    :ivar ~.id: The unique identifier for the simulation engine.
+    :vartype ~.id: str
+    :ivar ~.description: A short description of this engine.
+    :vartype ~.description: str
     :ivar citations: A list of citations for the simulation engine.
     :vartype citations: list of :class:`.Citation`
     """

stdpopsim/models.py

@@ -114,10 +114,10 @@ class Population(object):
     """
     Class recording metadata representing a population in a simulation.
 
-    :ivar id: The id of the population.
-    :vartype id: str
-    :ivar description: a short description of the population
-    :vartype description: str
+    :ivar ~.id: The id of the population.
+    :vartype ~.id: str
+    :ivar ~.description: a short description of the population
+    :vartype ~.description: str
     :ivar sampling_time: an integer value which indicates how many
         generations prior to the present individuals should samples should
         be drawn from this population. If `None`, sampling not allowed from this
@@ -149,13 +149,13 @@ class DemographicModel(object):
     This class is indended to be used by model implementors. To instead
     obtain a pre-specified model, see :class:`Species.get_demographic_model`.
 
-    :ivar id: The unique identifier for this model. DemographicModel IDs should be
+    :ivar ~.id: The unique identifier for this model. DemographicModel IDs should be
         short and memorable, perhaps as an abbreviation of the model's name.
-    :vartype id: str
-    :ivar description: A short description of this model as it would be used in
+    :vartype ~.id: str
+    :ivar ~.description: A short description of this model as it would be used in
         written text, e.g., "Three population Out-of-Africa". This should
         describe the model itself and not contain author or year information.
-    :vartype description: str
+    :vartype ~.description: str
     :ivar long_description: A concise, but detailed, summary of the model.
     :vartype long_description: str
     :ivar generation_time: Mean inter-generation interval, in years.

stdpopsim/species.py

@@ -61,14 +61,14 @@ class Species(object):
     """
     Class representing a species in the catalog.
 
-    :ivar id: The unique identifier for this species. The species ID is
+    :ivar ~.id: The unique identifier for this species. The species ID is
         the three first letters of the genus name followed by the first
         three letters of the species name, and does not
         contain any spaces or punctuation. The usual scheme is to
         use the first three letters of the genus and species (similar to the
         approach used in the UCSC genome browser), e.g., "HomSap"
         is the ID for Homo Sapiens.
-    :vartype id: str
+    :vartype ~.id: str
     :ivar name: The full name of this species in binominal nomenclature as
         it would be used in written text, e.g., "Homo sapiens".
     :vartype name: str