changes to Errors and Python doc pages

doc/src/Python_install.txt

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+"Higher level section"_Python.html - "LAMMPS WWW Site"_lws - "LAMMPS
+Documentation"_ld - "LAMMPS Commands"_lc :c
+
+:link(lws,http://lammps.sandia.gov)
+:link(ld,Manual.html)
+:link(lc,Section_commands.html#comm)
+
+:line
+
+Installing LAMMPS in Python :h3
+
+For Python to invoke LAMMPS, there are 2 files it needs to know about:
+
+python/lammps.py
+src/liblammps.so :ul
+
+Lammps.py is the Python wrapper on the LAMMPS library interface.
+Liblammps.so is the shared LAMMPS library that Python loads, as
+described above.
+
+You can insure Python can find these files in one of two ways:
+
+set two environment variables
+run the python/install.py script :ul
+
+If you set the paths to these files as environment variables, you only
+have to do it once.  For the csh or tcsh shells, add something like
+this to your ~/.cshrc file, one line for each of the two files:
+
+setenv PYTHONPATH $\{PYTHONPATH\}:/home/sjplimp/lammps/python
+setenv LD_LIBRARY_PATH $\{LD_LIBRARY_PATH\}:/home/sjplimp/lammps/src :pre
+
+If you use the python/install.py script, you need to invoke it every
+time you rebuild LAMMPS (as a shared library) or make changes to the
+python/lammps.py file.
+
+You can invoke install.py from the python directory as
+
+% python install.py \[libdir\] \[pydir\] :pre
+
+The optional libdir is where to copy the LAMMPS shared library to; the
+default is /usr/local/lib.  The optional pydir is where to copy the
+lammps.py file to; the default is the site-packages directory of the
+version of Python that is running the install script.
+
+Note that libdir must be a location that is in your default
+LD_LIBRARY_PATH, like /usr/local/lib or /usr/lib.  And pydir must be a
+location that Python looks in by default for imported modules, like
+its site-packages dir.  If you want to copy these files to
+non-standard locations, such as within your own user space, you will
+need to set your PYTHONPATH and LD_LIBRARY_PATH environment variables
+accordingly, as above.
+
+If the install.py script does not allow you to copy files into system
+directories, prefix the python command with "sudo".  If you do this,
+make sure that the Python that root runs is the same as the Python you
+run.  E.g. you may need to do something like
+
+% sudo /usr/local/bin/python install.py \[libdir\] \[pydir\] :pre
+
+You can also invoke install.py from the make command in the src
+directory as
+
+% make install-python :pre
+
+In this mode you cannot append optional arguments.  Again, you may
+need to prefix this with "sudo".  In this mode you cannot control
+which Python is invoked by root.
+
+Note that if you want Python to be able to load different versions of
+the LAMMPS shared library (see "this section"_#py_5 below), you will
+need to manually copy files like liblammps_g++.so into the appropriate
+system directory.  This is not needed if you set the LD_LIBRARY_PATH
+environment variable as described above.