Python: len(series)

[ax3l]

We should consider if we want to overwrite the __len__/len() of openPMD_api.Series.

Currently, it (probably) returns the number of attributes as for all Attributable objects in openPMD.

It might be maybe a bit more intuitive if it returns the len(series.iterations) instead.

First seen by @RTSandberg .

Refs.:

• https://docs.python.org/3/library/functions.html#len

[franzpoeschel]

We should generally do an overhaul of our object model in Python. The string representations of many classes are counterintuitive and have led to confusion, e.g. series.iterations printed will look as if it is empty:

> s.iterations
<openPMD.Attributable with '0' attributes>
>>> s.iterations[0]
<openPMD.Iteration at t = '0.000000e+00 s'>

Also, now that we have done a redesign of our C++ object model, we can introduce proper semantics for equality comparison. I recently wanted to use record components as keys in a Python dict, this currently leads to crashes. --> #1658