There were several places in the code that had to check the type of an
`UnknownArrayHandle` and conditionally get or copy that array. This code
is simplified by using `ArrayCopyShallowIfPossible`.
`VaraintArrayHandle` has been replaced by `UnknownArrayHandle` and
`UncertainArrayHandle`. Officially make it deprecated and point users to
the new implementations.
C++11 introduced the `std::vector::data()` method. In addition to being
more syntatically pleasing, it should correctly handle the condition
when the `std::vector` is size 0 and therefore has no real pointer.
(Expressions like `&v[0]` are undefined when the `vector` is empty.)
Most of this code is not templated methods. Rather, it implements over
several types to call templated functions, which creates quite a bit of
code. Rather than have all code using a reader recompile the code, just
compile it once and put it in a library.
