Improve performance of axom::Array::push_back

[kennyweiss]

In #1469, we added some initial benchmarking of axom::Array w.r.t. std::vector (see also #287),
and observed that axom::Array was slower for push_back and emplace_back operations on native types.

Initial profiling using hpctoolkit did not reveal any low-hanging fruit.
Our assumption is that the performance might be due to function call overhead, e.g. for functions that are not inlined.

@publixsubfan noticed that axom::Array::emplace() calls the virtual function Array::updateNumElements(), and axom::Array has several other virtual functions.