Python Series/Iteration: Fix Length (#1659)

* Python Series/Iteration: Fix Length

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
```

* Python: Remove `Attributable.__len__`

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

NEWS.rst

@@ -16,6 +16,9 @@ Note that ADIOS2 does not support compression in BP3 files.
 CMake 3.22.0 is now the minimally supported version for CMake.
 pybind11 2.12.0 is now the minimally supported version for Python support.
 
+The ``len(...)`` of many classes has been reworked for consistency and returns now the number of entries (iterations, record components, etc.).
+Previously, this sporadically returned the number of attributes, which is better queried via ``len(<object>.attributes)``.
+
 
 0.15.0
 ------

include/openPMD/binding/python/Container.H

@@ -85,6 +85,12 @@ declare_container(py::handle scope, std::string const &name)
         // keep container alive while iterator exists
         py::keep_alive<0, 1>());
 
+    // overwrite to avoid that the __len__ of Attributable is used
+    cl.def(
+        "__len__",
+        [](const Map &m) { return m.size(); },
+        "Number of elements in the container to iterate.");
+
     cl.def("__repr__", [name](Map const &m) {
         std::stringstream stream;
         stream << "<openPMD." << name << " with ";

src/binding/python/Attributable.cpp

@@ -629,8 +629,6 @@ void init_Attributable(py::module &m)
         .def("delete_attribute", &Attributable::deleteAttribute)
         .def("contains_attribute", &Attributable::containsAttribute)
 
-        .def("__len__", &Attributable::numAttributes)
-
         // @todo _ipython_key_completions_ if we find a way to add a []
         // interface
 

src/binding/python/Series.cpp

@@ -252,6 +252,7 @@ For further details, refer to the non-MPI overload.
             )END")
 #endif
         .def("__bool__", &Series::operator bool)
+        .def("__len__", [](Series const &s) { return s.iterations.size(); })
         .def(
             "__repr__",
             [](Series const &s) {