Allow different demand flexibility duration parameters for different demand sectors/load zones(buses)/intervals

# :rocket:

- [ ] Is your feature request essential for your project?

### Describe the workflow you want to enable
In the current demand flexibility setup, we allow user to specify a single duration parameter, i.e. over how long of a period can the demand be shifted, within every simulation interval for all load zones. However, once the parameter is set, it applies to all demand sectors, all intervals and load zones. Ideally, I wish I could specify different duration parameters for different intervals, load zones (buses) and even demand sectors. This will maximize the flexibility of modeling demand shifting (shedding?) behavior.

### Additional context
This feature not essential for the time being but good have, which can be implemented after #172 .