Add Attribute::getOptional<T>() and use to add some more dynamic datatype conversions at read time

[franzpoeschel]

Currently, we have Attribute::get<T>() that throws a std::runtime_error() if no conversion is possible.
Adding Attribute::getOptional<T>() is helpful to check if a conversion is possible without having to resort to catching exceptions.

This PR also uses that new API method while reading a Series in. This became necessary in #1277 from where I have extracted this PR (that PR adds a less verbose mode to the JSON backend that does not explicitly annotate datatypes and might hence report different numerical types at read times).

[franzpoeschel]

We could remove getCast here in a similar way. Slightly API-breaking though since this is in a public header.

[franzpoeschel]

I did this API break now. All implementation details are now moved to the detail namespace.

[ax3l]

Please add in Python, too (pybind11 returns py:None for std::nullopt)

[franzpoeschel]

Python does not use this call at all:

        // C++ pass-through API: Getter
        .def(
            "get_attribute",
            [](Attributable &attr, std::string const &key) {
                auto v = attr.getAttribute(key);
                return v.getResource();
                // TODO instead of returning lists, return all arrays (ndim > 0)
                // as numpy arrays?
            })

There is no explicit type requesting as get<T>() does, in Python you just get the type that there is

[ax3l]

Yes, I was wondering if we want to add such an option in Python to cast?