Define VWR_SIZE_TYPE to the type you want to use as the template
index type. It defaults to std::size_t so if you don't do anything
you still get the old behaviour.
Also drop the std::index_range in vector_cast in favour of the custom
bt::number_range, which is already used elsewhere in the code.
Casting conditions got clarified.
User is now able to say he doesn't care if casting a wrapper on a
type larger than just the coordinates will stomp on extra data
from the casted type.
Add vector_type to is_vec, because vector_type is private
and it couldn't be used in cast. Besides if not is_vec, you can't
expect to find vector_type. The conditional in the cast return type
works like if V2 is not a Vec, you can just use V as the vector_type,
anyways the first is_vec is false so the whole condition is going to
be false anyways, even if it results that V == V.
