Add simple examples of wavefunctions

Add examples of wavefunctions without plotting.

```julia
using Antique
HO = HarmonicOscillator(k=1.0, m=1.0, ℏ=1.0)
```

```julia
julia> ψ(HO, 0.0)
0.7511255444649425

julia> ψ(HO, 0.0, n=0)
0.7511255444649425

julia> ψ(HO, 0.0, n=1)
0.0

julia> ψ(HO, 0.0, n=3)
0.0
```