Rename axom::Array::set to axom::Array::assign for better conformance to std::vector

[kennyweiss]

It was pointed out in #881 that axom::Array is missing an assign() method, but has a set() method with similar behavior.

@kanye-quest suggested renaming the latter to the former

• std::vector::assign -- https://en.cppreference.com/w/cpp/container/vector/assign
• axom::Array::set -- https://github.com/LLNL/axom/blob/74b8b9133a9f233fa51ae794a23ceb6b70690551/src/axom/core/Array.hpp#L355-L367