Convenience functions for element-wise operations

[lp2]

Eigen does not allow syntax such as
double a = lngamma(b).sum();
where b is a matrix. Instead, one has to write a for loop:
double a = 0;
for (int i = 0; ...) {a += lngamma(b(i));}

It would be useful to have convenience functions for
gamma
log
lngamma
psi
exp