This works out the mean of all the (parametric / nonparametric) delay
distributions combined in the passed `dist_spec()`

.

## Usage

```
# S3 method for dist_spec
mean(x, ...)
```

## Examples

```
# A fixed lognormal distribution with mean 5 and sd 1.
lognormal <- dist_spec(
mean = 5, sd = 1, max = 20, distribution = "lognormal"
)
mean(lognormal)
#> [1] 14.60932
# An uncertain gamma distribution with mean 3 and sd 2
gamma <- dist_spec(
mean = 3, sd = 2, mean_sd = 0.5, sd_sd = 0.5, max = 20,
distribution = "gamma"
)
mean(gamma)
#> [1] 3
# The mean of the sum of two distributions
mean(lognormal + gamma)
#> [1] 14.60932 3.00000
```