Rewrote mass balance algorythm

Corrected several mistakes. I also believe it is more explicit now.
Enabled 'big' (intermediate calculations) output

Using big=True will cause the PDD model to output the number of PDD, snow accumulation, melt, runoff, and surface mass balance.
Using a dictionary for big output

I found the tuple unpractical for the GRASS script.
Made 'big' output usable in GRASS GIS

Output maps can be specified individually for each variable. No output is required.