add some documentation for the programmer guide and doxygen decorations

doc/src/Developer_utils.rst

@@ -292,6 +292,50 @@ This code example should produce the following output:
 
 ----------
 
+
+Argument parsing classes
+---------------------------
+
+The purpose of argument parsing classes it to simplify and unify how
+arguments of commands in LAMMPS are parsed and to make abstractions of
+repetitive tasks.
+
+The :cpp:class:`LAMMPS_NS::ArgInfo` class provides an abstraction
+for parsing references to compute or fix styles or variables. These
+would start with a "c\_", "f\_", "v\_" followed by the ID or name of
+than instance and may be postfixed with one or two array indices
+"[<number>]" with numbers > 0.
+
+A typical code segment would look like this:
+
+.. code-block:: C++
+   :caption: Usage example for ArgInfo class
+
+   int nvalues = 0;
+   for (iarg = 0; iarg < nargnew; iarg++) {
+     ArgInfo argi(arg[iarg]);
+
+     which[nvalues] = argi.get_type();
+     argindex[nvalues] = argi.get_index1();
+     ids[nvalues] = argi.copy_name();
+
+     if ((which[nvalues] == ArgInfo::UNKNOWN)
+          || (which[nvalues] == ArgInfo::NONE)
+          || (argi.get_dim() > 1))
+       error->all(FLERR,"Illegal compute XXX command");
+
+     nvalues++;
+   }
+
+----------
+
+.. doxygenclass:: LAMMPS_NS::ArgInfo
+   :project: progguide
+   :members:
+
+
+----------
+
 File reader classes
 -------------------