If the `len(...)` intrinsic for series and iterations.

```py
import openpmd_api as io

s = io.Series("build/samples/git-sample/3d-bp4/example-3d-bp4_%T.bp", io.Access.read_only)

s
<openPMD.Series at 'build/samples/git-sample/3d-bp4/example-3d-bp4_%T.bp' with 2 iteration(s) and 10 attributes>

s.iterations
<openPMD.Iteration_Container with 2 entries and 0 attribute(s)>

len(s)
2

len(s.iterations)
2
```

For consistency because its only a mixin class.
Write migration guide for breaking change.