Installs sparsehash when the external sparsehash package is found

[yurivict]

Here: https://github.com/LLNL/axom/blob/develop/src/thirdparty/CMakeLists.txt#L184

Despite configure saying that sparsehash is found:

-- Sparsehash configured with '<functional>' header

[rhornung67]

We should update sparsehash at the same time this is addressed.

[nselliott]

If I understand the request correctly, it would be to make it possible to have headers from an external installation of sparsehash override the headers that we distribute. This would be useful to users that already maintain a sparsehash installation that they use elsewhere in their application. That would mean creating an optional CMake symbol SPARSEHASH_DIR and writing the necessary CMake code to point to headers in $(SPARSEHASH_DIR)/include when a user provides an installation path. Is that the way to go?

[yurivict]

Sparsehash is available as a package almost on all systems: https://repology.org/project/sparsehash/versions

There should be no need to distribute it, you should just require the external sparsehash package.

[white238]

This is now guarded by the axom namespace and won't conflict with any users downstream.