invmod_RD: Invsere modulus for RDD
In soonwookwon/rdadapt: What the Package Does (One Line, Title Case)

Calculates the inverse modulus for the RDD problem.

1 2	invmod_RD(b, gamma, C, Xt, Xc, mon_ind, sigma_t = 1, sigma_c = 1, swap = FALSE)

`b`	point where the inverse modulus is evaluated at
`gamma`	length two vector of gammas ((γ_1, γ_2)')
`C`	length two vector of Cs ((C_1, C_2)')
`Xt`	n_t \times k design matrix for the treated units
`Xc`	n_c \times k design matrix for the control units
`mon_ind`	indice of the monotone variables
`sigma_t`	standard deviation of the error term for the treated units (either length 1 or n_t)
`sigma_c`	standard deviation of the error term for the control units
`swap`	indicator of whether to swap the parameter spaces