Also add STL option to the EB section (exa/docs!68)

Constructing Embedded Boundaries in MFIX-Exa

============================================

MFIX-Exa allows 3 approaches to specify embedded boundaries (EBs).

MFIX-Exa allows 4 approaches to specify embedded boundaries (EBs).

1. Using native AMReX cpp functions.

2. Using CSG file format.

3. Using planar walls.

4. Using STL file format.

There are also two parameters (specified in the ``inputs`` file) that influence

the level-set creation:

Alternatively, the constructive solid geometry can be created using

OpenSCAD's CSG file format by installing the ``csg-eb`` library.

To use this option, MFIX-Exa must be built with the flag ``MFIX_CSG=TRUE``

and the option ``mfix.geometry_filename`` must be set in the ``inputs``.

See the `CSG-EB repository`_  for more details about this format.

and the options ``mfix.geom_type = csg`` and ``mfix.geometry_filename``

must be set in the ``inputs``. See the `CSG-EB repository`_  for more

details about this format.

Planar Walls

any no-slip or free-slip boundary conditions are expressed as EB walls.

STL File Format

***************

A standard triangle language (STL) file created using numerous

CAD programs can also specify the EB geometry. To use this method,

the options ``mfix.geom_type = stl`` and ``mfix.geometry_filename``

must be set in the ``inputs``.

How MFIX-Exa Constructs the EB Geometry

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