Out-of-source build improvement

[DanW97]

On the main installation instructions where it references an out-of-source build (../build instead of ../src), the installation script assumes that you have built liggghts in the latter directory and so complains that it can't find the .so object.

It would be helpful to other users to include the need to alter line 41 from

str = "cp ../src/libliggghts.so %s" % libdir

to

str = "cp ../$INSTALL_DIR$/libliggghts.so %s" % libdir

if an out-of-source build is performed.

Mentioning also that the user needs to set the PYTHONPATH and LD_LIBRARY_PATH accordingly would be massively useful also.