changes to Errors and Python doc pages

doc/src/Python_overview.txt

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+"Previous Section"_Examples.html - "LAMMPS WWW Site"_lws -
+"LAMMPS Documentation"_ld - "LAMMPS Commands"_lc - "Next
+Section"_Tools.html :c
+
+:link(lws,http://lammps.sandia.gov)
+:link(ld,Manual.html)
+:link(lc,Commands.html#comm)
+
+:line
+
+Overview of Python and LAMMPS :h3
+
+LAMMPS can work together with Python in three ways.  First, Python can
+wrap LAMMPS through the its "library interface"_Howto_library.html, so
+that a Python script can create one or more instances of LAMMPS and
+launch one or more simulations.  In Python lingo, this is "extending"
+Python with LAMMPS.
+
+Second, a lower-level Python interface can be used indirectly through
+provided PyLammps and IPyLammps wrapper classes, written in Python.
+These wrappers try to simplify the usage of LAMMPS in Python by
+providing an object-based interface to common LAMMPS functionality.
+They also reduces the amount of code necessary to parameterize LAMMPS
+scripts through Python and make variables and computes directly
+accessible.
+
+Third, LAMMPS can use the Python interpreter, so that a LAMMPS
+input script can invoke Python code directly, and pass information
+back-and-forth between the input script and Python functions you
+write.  This Python code can also callback to LAMMPS to query or change
+its attributes.  In Python lingo, this is "embedding" Python in
+LAMMPS.  When used in this mode, Python can perform operations that
+the simple LAMMPS input script syntax cannot.
+
+