print methods should invisibly return the object

In base R,

```r
foo <- print(2)
#> [1] 2
foo
#> [1] 2
```

but

```r
env <- print(simmer())
#> simmer environment: anonymous | now: 0 | next: 
env
#> NULL

traj <- print(trajectory())
#> trajectory: anonymous, 0 activities
traj
#> NULL
```

etc.