change link to multigrid notebook (#182)

we split up the old notebook, so this points to the basic example
now

Author: zingale

README.md

@@ -105,10 +105,10 @@ on the grid are described in a jupyter notebook:
 https://github.com/python-hydro/pyro2/blob/main/pyro/mesh/mesh-examples.ipynb
 
 
-Many of the methods here rely on multigrid.  The multigrid solver is
+Many of the methods here rely on multigrid.  The basic multigrid solver is
 demonstrated in the juputer notebook:
 
-https://github.com/python-hydro/pyro2/blob/main/pyro/multigrid/multigrid-examples.ipynb
+https://github.com/python-hydro/pyro2/blob/main/pyro/multigrid/multigrid-constant-coefficients.ipynb
 
 
 ## Solvers
@@ -251,7 +251,7 @@ with their data.
   There is a set of notes that describe the background and details of the
   algorithms that pyro implements:
 
-  http://bender.astro.sunysb.edu/hydro_by_example/CompHydroTutorial.pdf
+  http://open-astrophysics-bookshelf.github.io/numerical_exercises/
 
   The source for these notes is also available on github:
 

docs/source/multigrid_basics.rst

@@ -4,7 +4,7 @@ Multigrid Class Overview
 pyro solves elliptic problems (like Laplace's equation or Poisson's
 equation) through multigrid. This accelerates the convergence of
 simple relaxation by moving the solution down and up through a series
-of grids. Chapter 9 of the `pdf notes <http://bender.astro.sunysb.edu/hydro_by_example/CompHydroTutorial.pdf>`_ gives an introduction to solving elliptic equations, including multigrid.
+of grids. Chapter 9 of the `pdf notes <http://open-astrophysics-bookshelf.github.io/numerical_exercises/CompHydroTutorial.pdf>`_ gives an introduction to solving elliptic equations, including multigrid.
 
 There are three solvers:
 

docs/source/notes.rst

@@ -6,7 +6,7 @@ Notes on the numerical methods
 Detailed discussions and derivations of the numerical methods used in
 pyro are given in the set of notes `Introduction to Computational
 Astrophysical Hydrodynamics
-<http://bender.astro.sunysb.edu/hydro_by_example/CompHydroTutorial.pdf>`_,
+<http://open-astrophysics-bookshelf.github.io/numerical_exercises/CompHydroTutorial.pdf>`_,
 part of the `Open Astrophysics Bookshelf
 <https://github.com/Open-Astrophysics-Bookshelf>`_.