document the build system

Author: zingale

Docs/source/build_system.rst

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+************
+Build System
+************
+
+Microphysics leverages the AMReX build system.  GNU make is the
+primary build system, but CMake is partially supported.  Here we focus
+on the GNU make build system.
+
+.. tip::
+
+   All of the build variables supported by the AMReX build system can
+   be used with Microphysics, for example:
+
+   * ``COMP`` : the compiler system to use
+   * ``USE_CUDA`` : build for NVIDIA GPUs with CUDA
+   * ``USE_HIP`` : build for AMD GPUs with HIP/ROCm
+
+   See the `AMReX GNU Make Docs
+   <https://amrex-codes.github.io/amrex/docs_html/BuildingAMReX.html>`_
+
+There are two main makefile stubs:
+
+* ``Make.Microphysics``
+
+* ``Make.Microphysics_extern``
+
+additionally directories have their own ``Make.package`` files that specify
+the files needed for the build.
+
+
+AMREX_HOME
+MICROPHYSICS_HOME
+
+
+Controling choice of physics
+============================
+
+USE_CONDUCTIVITY
+USE_NEUTRINOS
+USE_SCREENING
+USE_RATES
+USE_REACT
+USE_NONAKA_PLOT
+
+USE_SIMPLIFIED_SDC
+USE_TRUE_SDC
+
+SCREEN_METHOD
+
+USE_AUX_THERMO
+
+EOS_DIR
+OPACITY_DIR
+NETWORK_DIR
+CONDUCTIVITY_DIR
+
+GENERAL_NET_INPUTS
+NETWORK_INPUTS
+
+USE_NSE_TABLE
+USE_NET_NET
+
+USE_RAD
+
+Targets
+=======
+
+nettables
+table
+nsetable
+build_status
+test_extern_params
+net_prop_debug
+
+clean
+
+
+runtime parameters
+
+buildInfo
+
+
+Querying the build variables
+============================