Construct halos from imports

[JStewart28]

Moved Collector implementation into a Halo that is constructed by specifying ids and ranks to import data from. (See PR #817. This PR closes PR #817)

An import-based Halo is used when you know who you are receiving from but not who you are sending to.

Additions:

1. Added Cabana::Export and Cabana::Import type tags.
2. A Cabana::Halo is now typed on MemorySpace and BuildType, where BuildType is either Cabana::Export or Cabana::Import. (Default Cabana::Export for backwards compatibility)
3. createFromXXX functions in Cabana_CommunicationPlan.hpp are renamed and typed on Cabana::Export or Cabana::Import to support overriding.
4. Updated Halo test to test halos built from imports.
5. Added ImportHaloExchange example. Renamed HaloExchange example to ExportHaloExchange.
6. From PR #817: Added Cabana_INSTALL_TEST_EXECUTABLE CMake flag that is set to OFF by default.

I'd like to get your opinion on my design of addition 3. A Halo needs four different constructors based on the combination of import/export, topo/no topo, and overriding the createFromXXX functions in CommunicationPlan let me use less hard coding of function names in those constructors.

[JStewart28]

Looks close - can you remove the namespace commits (will make the update once we merge that much easier)? Will be easier to review

Yes. Just reverted them.

[JStewart28]

@streeve all checks have passed. Do you have any more comments or suggestions?