change Python.txt to Python_head.txt, adapt GPU package syntax

doc/src/Python_head.txt

@@ -0,0 +1,82 @@
+"Previous Section"_Modify.html - "LAMMPS WWW Site"_lws -
+"LAMMPS Documentation"_ld - "LAMMPS Commands"_lc - "Next
+Section"_Errors.html :c
+
+:link(lws,http://lammps.sandia.gov)
+:link(ld,Manual.html)
+:link(lc,Commands_all.html)
+
+:line
+
+Use Python with LAMMPS :h2
+
+These doc pages describe various ways that LAMMPS and Python can be
+used together.
+
+<!-- RST
+
+.. toctree::
+   :maxdepth: 1
+
+   Python_overview
+
+.. toctree::
+   :maxdepth: 1
+
+   Python_run
+   Python_shlib
+   Python_install
+   Python_mpi
+   Python_test
+   Python_library
+   Python_pylammps
+   Python_examples
+
+.. toctree::
+   :maxdepth: 1
+
+   Python_call
+
+END_RST -->
+
+<!-- HTML_ONLY -->
+
+"Overview of Python and LAMMPS"_Python_overview.html :all(b)
+
+"Run LAMMPS from Python"_Python_run.html
+"Build LAMMPS as a shared library"_Python_shlib.html
+"Install LAMMPS in Python"_Python_install.html
+"Extend Python to run in parallel"_Python_mpi.html
+"Test the Python/LAMMPS interface"_Python_test.html
+"Python library interface"_Python_library.html
+"PyLammps interface"_Python_pylammps.html
+"Example Python scripts that use LAMMPS"_Python_examples.html :all(b)
+
+"Call Python from a LAMMPS input script"_Python_call.html :all(b)
+
+<!-- END_HTML_ONLY -->
+
+If you're not familiar with "Python"_http://www.python.org, it's a
+powerful scripting and programming language which can do most
+everything that lower-level languages like C or C++ can do in fewer
+lines of code.  The only drawback is slower execution speed.  Python
+is also easy to use as a "glue" language to drive a program through
+its library interface, or to hook multiple pieces of software
+together, such as a simulation code plus a visualization tool, or to
+run a coupled multiscale or multiphysics model.
+
+See the "Howto_couple"_Howto_couple.html doc page for more ideas about
+coupling LAMMPS to other codes.  See the "Howto
+library"_Howto_library.html doc page for a description of the LAMMPS
+library interface provided in src/library.h and src/library.h.  That
+interface is exposed to Python either when calling LAMMPS from Python
+or when calling Python from a LAMMPS input script and then calling
+back to LAMMPS from Python code.  The library interface is designed to
+be easy to add funcionality to.  Thus the Python interface to LAMMPS
+is also easy to extend as well.
+
+If you create interesting Python scripts that run LAMMPS or
+interesting Python functions that can be called from a LAMMPS input
+script, that you think would be genearlly useful, please post them as
+a pull request to our "GitHub site"_https://github.com/lammps/lammps,
+and they can be added to the LAMMPS distribution or webpage.