Create Constraints for SMC

```
redist.constraint.helper(
constraints = "vra",
tgt_min = 0.55,
group_pop,
total_pop,
ndists,
nmmd,
strength_vra = 2500,
pow_vra = 1.5
)
```

- constraints
Vector of constraints to include. Currently only 'vra' implemented.

- tgt_min
Defaults to 0.55. If 'vra' included, the minority percent to encourage in each district.

- group_pop
A vector of populations for some subgroup of interest.

- total_pop
A vector containing the populations of each geographic unit.

- ndists
The total number of districts.

- nmmd
The number of majority minority districts to target for 'vra' constraint

- strength_vra
The strength of the 'vra' constraint. Defaults to 2500.

- pow_vra
The exponent for the 'vra' constraint. Defaults to 1.5.

list of lists for each constraint selected