new plot_shock_decomposition options

It would be useful to allow the preprocessor to take the following new options, both for plot_shock_decomposition and initial_condition_decomposition:

• diff: plots the first difference of the requested variable
• flip: flip the requested variables with sign change (e.g. useful to flip the concept of an exchange rate or trade balance or deficit/surplus variable without having to define it in model definition)

I have already provisions ready to push.