log likelihood and derivatives for exponential distribution

`llikExp(x, rate, full = FALSE)`

- x
variable that is distributed by exponential distribution

- rate
vector of rates.

- full
Add the data frame showing x, mean, sd as well as the fx and derivatives

data frame with `fx`

for the log pdf value of with `dRate`

that has the derivatives with respect to the `rate`

parameter
the observation time-point

```
llikExp(1, 1:3)
#> fx dRate
#> 1 -1.000000 0.0000000
#> 2 -1.306853 -0.5000000
#> 3 -1.901388 -0.6666667
llikExp(1, 1:3, full=TRUE)
#> x rate fx dRate
#> 1 1 1 -1.000000 0.0000000
#> 2 1 2 -1.306853 -0.5000000
#> 3 1 3 -1.901388 -0.6666667
```