Make hydro flexible

# :rocket:

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

### Describe the workflow you want to enable
I wish I could specify the `Pmin` of hydro plant as a percentage of `Pmax` in real time given hydro generation is associated with its profile. In current implementation, we set `Pmin` to be 95% (or 100% need to confirm) of `Pmax` (from profile for each hour) to mandate hydro not being "curtail-able". However, that's not the case in an Europe grid model, where hydro power is operated more flexibly. So in desired implementation, the user could specify which hydro plants have mandate schedule and which ones are flexible by specifying `Pmin` as a percentage of `Pmax`.

### Describe your proposed implementation, if applicable
Instead of hard coding it to be equality [here](https://github.com/Breakthrough-Energy/REISE.jl/blob/2a691045b088893c619814ec8a728e9cdd33660d/src/model.jl#L480), adding a user specified parameter of `Pmin` being a percentage of `Pmax` (profile number for each hour).

### Describe alternatives you've considered, if relevant
For now, we can simply relax the constraint [here](https://github.com/Breakthrough-Energy/REISE.jl/blob/2a691045b088893c619814ec8a728e9cdd33660d/src/model.jl#L480) to make hydro curtail-able similarly as wind and solar. We will need to check how different the power output of those hydro plants that we would like to mandate in the relaxed setup.

### Additional context
This issue is considered as fully addressed when we have the capability of having both mandate and flexible hydro co-exist.