* Add MPI_Comm data member when MPI is enabled; removed the MPI_Comm
from the derived class SidreDataCollection.
* Add method to get the associated MPI_Comm: GetComm().
* In the constructor, if the collection_name contains '/' split it into
prefix + name.
* Made constructor explicit to suppress implicit conversion from
std::string.
In class VisItDataCollection:
* Add constructor that takes MPI_Comm + collection name as input: this
constructor can be with the Load() method to load parallel meshes.