Expected value for the Moran coefficient of model residuals under the null hypothesis of no spatial autocorrelation.

`expected_mc(X, C)`

Chun, Yongwan and Griffith, Daniel A. (2013). Spatial statistics and geostatistics. Sage, p. 18.

- X
model matrix, including column of ones.

- C
Connectivity matrix.

Returns a numeric value.

```
data(georgia)
C <- shape2mat(georgia)
X <- model.matrix(~ ICE + college, georgia)
expected_mc(X, C)
```