`UnknownCellSet` is an updated replacement for `DynamicCellSet`. The
next step in the replacement is to change `DataSet` to use the new
class.
Also replaced `DynamicCellSet` with `UnknownCellSet` in a few
places where `DynamicCellSet.h` was not directly included (and
therefore now no longer included at all). This change would have
to be made at some point anyway.
`VaraintArrayHandle` has been replaced by `UnknownArrayHandle` and
`UncertainArrayHandle`. Officially make it deprecated and point users to
the new implementations.
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.
