Add details about fcompare in Debugging section

when a floating point exception occurs.  One can then examine those

files to track down the origin of the issue.

Writing a :cpp:`MultiFab` to disk with

Several other ways to look at the data include:

1) Writing a :cpp:`MultiFab` to disk with

.. highlight:: c++

    VisMF::Write(const FabArray<FArrayBox>& mf, const std::string& name);

and examining it with ``Amrvis`` (section :ref:`sec:amrvis` in the AMReX documeintation) 

can be helpful as well.  

and examining it with ``Amrvis`` (section :ref:`sec:amrvis` in the AMReX documentation).

You can also use the :cpp:`print_state` routine: 

2) You can also use the :cpp:`print_state` routine: 

.. highlight:: c++

which outputs the data for a single cell.

Valgrind is another useful debugging tool.  Note that for runs using

3) If you want to compare old and new plotfiles, 

.. highlight:: c++

::

    fcompare --infile1 plt00000_run1 --infile2 plt00000_run2 --diffvar u_g

will print out the maximum absolute and relative differences between the two plotfiles

for each variable and will also create a new plotfile "diffs" that contains the difference

in u_g (in this case) between the two plotfiles.

The :cpp:`fcompare` executable can be built in AMReX (go to amrex/Tools/Plotfile and type "make").

4) Valgrind is another useful debugging tool.  Note that for runs using

more than one MPI process, one can tell valgrind to output to different 

files for different processes.  For example,