adding instructions for obtaining AMReX-Hydro

Building MFiX-Exa with gmake 

If you want to use gmake to build MFiX_Exa, you will need to have already

cloned amrex into a local directory:

cloned amrex and AMReX-Hydro into a local directory:

.. code:: shell

    > git clone https://github.com/amrex-codes/amrex

    > git clone https://github.com/AMReX-Codes/AMReX-Hydro.git

Then

    > git clone http://mfix.netl.doe.gov/gitlab/exa/mfix.git

    > cd mfix/exec

Edit the GNUmakefile to set AMREX_HOME to be the path to the directory

where you have put amrex.  Other options that you can set include

Edit the GNUmakefile (or use an environment variable)

to set AMREX_HOME and AMREX_HYDRO_HOME

to be the path to the directories where you have put amrex

and AMReX-Hydro.  Other options that you can set include

+-----------------+------------------------------+------------------+-------------+

| Option name     | Description                  | Possible values  | Default     |