Adapt likelihoods init to check for sympy.

GPy/likelihoods/__init__.py

@@ -4,11 +4,19 @@ from gaussian import Gaussian
 from gamma import Gamma
 from poisson import Poisson
 from student_t import StudentT
-from sstudent_t import SstudentT
 from likelihood import Likelihood
 from mixed_noise import MixedNoise
-from symbolic import Symbolic
-from negative_binomial import Negative_binomial
-from skew_normal import Skew_normal
-from skew_exponential import Skew_exponential
-from null_category import Null_category
+# TODO need to fix this in a config file.
+try:
+    import sympy as sym
+    sympy_available=True
+except ImportError:
+    sympy_available=False
+if sympy_available:
+    # These are likelihoods that rely on symbolic.
+    from symbolic import Symbolic
+    from sstudent_t import SstudentT
+    from negative_binomial import Negative_binomial
+    from skew_normal import Skew_normal
+    from skew_exponential import Skew_exponential
+    from null_category import Null_category