adding XGC support

Some details on the XGC support:

• ADIS creates blocks that are different from the blocks that are read from ADIOS. The blocks are determined based on total number of planes and a default value for number of planes per block (set to 8, but the user can change it). The user needs to specify a number_of_planes object in the JSON and can optionally also set planes_per_block if they want to change the default value.
• XGC VTK-m files: It turns out that the XGC support is in VTK-m, with the various files having "Extrude" in the name. I was able to use CellSetExtrude as is, but needed to make changes for the coordinate system and for fields to support the ability to have PartitionedDataSets.
• There are two examples under examples/xgc (along with a readme explaining what's needed for the JSON file when reading XGC data). xgc.cxx reads all data from files, while xgc-sst.cxx reads mesh from file but the 3d field data using SST (which has been tested out with the VM). Also added a small 4-plane XGC dataset for test-xgc.cxx as well.

One last change which didn't end up being needed for the XGC support, but is probably still good to keep:

• Added ADIS_USE_MPI. Previously in some places ADIOS2_HAVE_MPI or USE_MPI were used. So now there's just one to use consistently across the code, examples, and tests.