Solve DID example resolves #2

[MatthieuStigler]

The problem in the example was due to the infamous problem of R dropping dimension from matrix to scalar. This fix jsut adds drop=FALSE, so now the example runs.

library(MCPanel)
DID(matrix(c(1,2,3,4),2,2), matrix(c(1,1,1,0),2,2))
#>      [,1] [,2]
#> [1,]    1    3
#> [2,]    2    4

^{Created on 2021-08-17 by the reprex package (v2.0.1)}