Extend value to arrays when using JSON pointers

[nlohmann]

As discovered here, value(json_pointer) throws an exception, when accessing an array. This deviates from the documented behavior:

try {
   return at(ptr);
} catch(out_of_range) {
   return default_value;
}

Allow value(json_pointer) to work with arrays, update documentation, and update unit tests.

Replaces stale #3759