Feature/excel template

Icings on this cake which still need to be added:
1) frontend - add fields for economic start year and end year (they should be called econ_datastart and econ_dataend)

2) backend - predefined parameter abbreviations should be added for predefined parameters, and used instead of autogenerated parameter abbreviations

3) some refactoring in optima_workbook.py (to make it completely configurable) 

4) documenting what optima_workbook.py does (before I forgot :) )

for the rest, it seems to work (at least on my machine  ;-) )