[UMBRELLA] Overhaul of output data mechanism

[jamesturner246]

Requires an overhaul to the output data file generating mechanism (the analysis module).

Some requests:

• Add ability to group by discrete variables, instead of just age and gender (e.g. income, age, gender) #376
• Add ability to group by continuous variable ranges (e.g. sodium, BMI, ...) #377
• StdDev for YLL, YLD, DALY. #439
• arbitrary stats columns
• ability to enable outputting on a per-individual, in addition to the usual per-group #379
• Decide on the final data structure of the analysis module #441

Please add more as you think of them.