Remove stochkit initial condition support for numeric ICs (#405)

Raw numeric intial conditions, as opposed to those wrapped in a
parameter or constant expression, are not allowed in PySB.
This removes a superfluous check for this condition in the
StochKit exporter.

pysb/export/stochkit.py

@@ -165,9 +165,7 @@ def export(self, initials=None, param_values=None):
                 if si is None:
                     raise IndexError("Species not found in model: %s" %
                                      repr(cp))
-                if isinstance(ic.value, (int, float)):
-                    value = ic.value
-                elif ic.value in self.model.parameters:
+                if ic.value in self.model.parameters:
                     pi = self.model.parameters.index(ic.value)
                     value = param_values[pi]
                 elif ic.value in self.model.expressions: