docstrings in python version / Python documentation

**Is your feature request related to a problem? If so, please describe.**
If one interactively uses openpmd-api in python e.g. via ipython or jupyter, one might need to look up the usage via reading the docstrings as e.g. below

Python:
```python
import openpmd_api as io
io.Series?
```

results in a less-user-friendly output
```
Docstring:
Init docstring:
__init__(*args, **kwargs)
Overloaded function.

1. __init__(self: openpmd_api.openpmd_api_cxx.Series, filepath: str, access: openpmd_api.openpmd_api_cxx.Access, options: str = '{}') -> None

2. __init__(self: openpmd_api.openpmd_api_cxx.Series, filepath: str, access: openpmd_api.openpmd_api_cxx.Access, mpi_communicator: object, options: str = '{}') -> None
File: /trinity/shared/pkg/filelib/openpmd/0.14.4/gcc/7.3.0/openmpi/4.1.1-no-cuda/lib64/python3.6/site-packages/openpmd_api/openpmd_api_cxx.cpython-36m-x86_64-linux-gnu.so
Type: pybind11_type
Subclasses:
```

**Describe the solution you'd like**
It would be fantastic if one could have a more readably docstring for the openPMD-api python module.

**Describe alternatives you've considered**
Currently the best way (for me) is to follow the examples.

**Additional context**
This is not urgent, but it would enhance productivity if one does not have to look for the right example or to scroll through the source code to figure out how to use the python version.