* Fix issue with HDF5_VOL_ALLOW_EXTERNAL CMake variable
* Add initial API test workflow
* Initialize parallel testing with MPI_THREAD_MULTIPLE when testing API
* Add CMake variable to allow specifying a VOL connector's package name
* Remove call to MPI_Init in serial API tests
While previously necessary, it now interferes with VOL connectors that
may need to be initialized with MPI_THREAD_MULTIPLE