* resize_null() methods for PtrList variants
  - for cases where an existing PtrList needs a specific size and
    but not retain any existing entries.

    Eg,
        ptrs.resize_null(100);

    vs.   ptrs.free();     ptr.resize(100);
    or    ptr.resize(100); ptrs.free();

* remove stored pointer before emplacing PtrList elements
  - may reduce memory peaks

* STYLE: static_cast of (nullptr) instead of reinterpret_cast of (0)

* COMP: implement emplace_set() for PtrDynList
  - previously missing, which meant it would have leaked through to the
    underlying PtrList definition

* emplace methods for autoPtr, refPtr, tmp
  - applies reset() with forwarding arguments.
    For example,

        tmp<GeoField> tfld = ...;

    later...

        tfld.emplace(io, mesh);

    vs.
        tfld.reset(new GeoField(io, mesh));
    or
        tfld.reset(tmp<GeoField>::New(io, mesh));

    The emplace() obviously has reduced typing, but also allows the
    existing stored pointer to be deleted *before* creating its
    replacement (reduces memory peaks).