Some improvements to python example scripts

latex_invoices.py and str_methods.py with some adjustments
latex_invoices:
- IPython embedded Shell compatibility in Version 2
- Some adjustments
str_methods:
- correctly prints out results of Entry.GetInvPrice(). Before it had been necessary to use the "instance" construct which is no loner necessary and actually not wnorking anymore

removal of some unneeded spaces