Updates to documentation. Replace XML parsing section with user input changes

section.

docs/source/devguide/user-input.rst

@@ -0,0 +1,62 @@
+.. _devguide_user_input:
+
+=========================
+Making User Input Changes
+=========================
+
+Users are encouraged to use OpenMC's :ref:`pythonapi` to build XML files that
+the OpenMC solver then reads during the initialization phase. Thus, to modify,
+add, or remove user input options, changes must be made both within the Python
+API and the Fortran source that reads XML files produced by the Python API. The
+following steps should be followed to make changes to user input:
+
+1. Determine the Python class you need to change. For example, if you are adding
+   a new setting, you probably want to change the :class:`openmc.Settings`
+   class. If you are adding a new surface type, you would need to create a
+   subclass of :class:`openmc.Surface`.
+
+2. To add a new option, the class will need a `property attribute`_. For
+   example, if you wanted to add a "fast_mode" setting, you would need two
+   methods that look like:
+
+   .. code-block:: python
+
+      @property
+      def fast_mode(self):
+          ...
+
+      @fast_mode.setter
+      def fast_mode(self, fast_mode):
+          ...
+
+3. Make sure that when an instance of the class is exported to XML (usually
+   through a ``export_to_xml()`` or ``to_xml_element()`` method), a new element
+   is written to the appropriate file. OpenMC uses the
+   :mod:`xml.etree.ElementTree` API, so refer to the documentation of that
+   module for guidance on creating elements/attributes.
+
+4. Make sure that your input can be categorized as one of the datatypes from
+   `XML Schema Part 2`_ and that parsing of the data appropriately reflects
+   this. For example, for a boolean_ value, true can be represented either by
+   "true" or by "1".
+
+5. Now that you're done with the Python side, you need to make modifications to
+   the Fortran codebase. Make appropriate changes in the `input_xml module`_ to
+   read your new user input. You should use procedures and types defined by the
+   `xml_interface module`_.
+
+6. If you've made changes in the geometry or materials, make sure they are
+   written out to the statepoint or summary files and that the
+   :class:`openmc.StatePoint` and :class:`openmc.Summary` classes read them in.
+
+For most user input additions and changes, it is simple enough to follow a
+"monkey see, monkey do" approach. When in doubt, contact your nearest OpenMC
+developer or send a message to the `developers mailing list`_.
+
+
+.. _property attribute: https://docs.python.org/3.6/library/functions.html#property
+.. _XML Schema Part 2: http://www.w3.org/TR/xmlschema-2/
+.. _boolean: http://www.w3.org/TR/xmlschema-2/#boolean
+.. _xml_interface module: https://github.com/mit-crpg/openmc/blob/develop/src/xml_interface.F90
+.. _input_xml module: https://github.com/mit-crpg/openmc/blob/develop/src/input_xml.F90
+.. _developers mailing list: https://groups.google.com/forum/?fromgroups=#!forum/openmc-dev