Builds a design with `N_groups`

groups each containing `N_i_group`

individuals.
Potential outcomes exhibit spillovers: if any individual in a group receives treatment,
the effect is spread equally among members of the group.

## Arguments

- N_groups
An integer. Number of groups.

- N_i_group
Number of units in each group. Can be scalar or vector of length

`N_groups`

.- sd_i
A nonnegative number. Standard deviation of individual-level shock.

- gamma
A number. Parameter that controls whether spillovers within groups substitute or complement each other. See `Details`.

- args_to_fix
A character vector. Names of arguments to be args_to_fix in design.

## Details

Parameter `gamma`

controls interactions between spillover effects.For `gamma`

=1 for every $1 given to a member of a group, each member receives $1*`N_i_group`

no matter how many others are already treated.
For `gamma`

>1 (<1) for every $1 given to a member of a group, each member receives an amount that depends negatively (positively) on the number already treated.

The default estimand is the average difference across subjects between no one treated and only that subject treated.