add docstrings to the Burgers problems (#277)

Author: zingale

pyro/burgers/problems/converge.py

@@ -1,3 +1,6 @@
+"""Setup a smooth velocity field (each component is an exponential).  This can
+be used to test convergence of the solver."""
+
 import numpy
 
 from pyro.util import msg

pyro/burgers/problems/test.py

@@ -1,3 +1,6 @@
+"""Create a shock moving diagonally (from lower-left to upper-right)
+through the domain."""
+
 from pyro.util import msg
 
 DEFAULT_INPUTS = "inputs.test"

pyro/burgers/problems/tophat.py

@@ -1,3 +1,5 @@
+"""Initialize a circular "tophat" region that will drive a shock."""
+
 from pyro.util import msg
 
 DEFAULT_INPUTS = "inputs.tophat"