Index Order Mappings

With the removal of [dataOrder](https://github.com/openPMD/openPMD-standard/pull/194) in openPMD 2.0 and the confusion it already adds in 1.0.0 we should provide an API that makes it easy to set/read attributes such as:

*base standard* attributes:
- `axisLabels`
- `gridSpacing`
- `gridGlobalOffset`
- `shape` of constant record components

and *ED-PIC* extension attributes:
- `fieldBoundary`
- `particleBoundary`

Attributes such as `axisLabels` should be an object and we could add an API of some kind of map. Otherwise it can be confusing if a user accesses low-level numpy attributes such as `.shape` which indices are then [in inverted order](https://github.com/openPMD/openPMD-standard/issues/189#issuecomment-384295697) if one is accessing from C/C++/Python.

So instead of
```C++
record.axisLabels = {"x", "y", "z"};
// ...
cout << record.axisLabels[0] << endl;
```

we should provide an access of that can not be confused or has a clear relation to a matrix' `A[k,j,i]` order.

Or we write setters/getters like
```C++
record.axisLabels.set({"x", "y", "z"}, indexOrder::fastToSlow);
// ...
cout << record.axisLabels.get(indexOrder::slowToFast) << endl;
// "z", "y", "x"
```

or we come up with something even more gentle.

Or we say "in the C++/Python API use the same order as your index order" and we invert it automatically on write according to the standard. They APIs don't need to match the low-level data, imho. Anyway, we did this in the past with libSplash and it was not necessarily removing confusion.