remove gresho from lm_atm + add docstring for bubble (#280)

it is not clear where the gresho initial conditions for low
Mach stratified flow came from.

Author: zingale

pyro/lm_atm/problems/__init__.py

@@ -1 +1 @@
-__all__ = ['bubble', 'gresho']
+__all__ = ['bubble']

pyro/lm_atm/problems/bubble.py

@@ -1,3 +1,8 @@
+"""Initialize a buoyant bubble in a hydrostatic atmosphere.  This is
+meant to be used to compare with the compressible solver bubble
+problem.
+"""
+
 import numpy
 
 from pyro.util import msg