avoid using absolute path
re-implement addBG -> getBackground

Details:
simplify the function (no need to loop over the json file line by
line). Specify the variation name as an argument, so that this
function can work also with the syst case. Clarify the fact that there
could be several backgrounds in the json file (rename 'BG' ->
'background1').

I still think that storing N backgrounds in the json is a bad idea.
Fbu does not care, and just wants one total background yield for now.
clarifying comment
move the 'array' string to the template

Details:
np.array type is not JSON serializable.
Also, this is just a list of numbers; the fact that is a np.array
should be hidden inside the model.
again, move 'array' to template
all the numbers in data/ as json

Details:
- moved all the numerical values into json files stored in data/
- clarified that we're reading them and formatting then as string, in
  order to fill the template
all configurables now as cmd-line

Details:
- All of the parameters that we want to configure in the model are now
  specified as cmd-line options.
- renamed some of the options
- print configuration when verbose
- some general cleanup