Use more than 1 MPI process in CI.

[matz-e]

Following up with HighFive more than one rank in MPI should work. During testing, I got a bunch of oversubscription error messages for two and more ranks:

#9 27.57 There are not enough slots available in the system to satisfy the 4
#9 27.57 slots that were requested by the application:
#9 27.57 
#9 27.57   /build/src/parquet2hdf5
#9 27.57 
#9 27.57 Either request fewer slots for your application, or make more slots
#9 27.57 available for use.
#9 27.57 
#9 27.57 A "slot" is the Open MPI term for an allocatable unit where we can
#9 27.57 launch a process.  The number of slots available are defined by the
#9 27.57 environment in which Open MPI processes are run:
#9 27.57 
#9 27.57   1. Hostfile, via "slots=N" clauses (N defaults to number of
#9 27.57      processor cores if not provided)
#9 27.57   2. The --host command line parameter, via a ":N" suffix on the
#9 27.57      hostname (N defaults to 1 if not provided)
#9 27.57   3. Resource manager (e.g., SLURM, PBS/Torque, LSF, etc.)
#9 27.57   4. If none of a hostfile, the --host command line parameter, or an
#9 27.57      RM is present, Open MPI defaults to the number of processor cores
#9 27.57 
#9 27.57 In all the above cases, if you want Open MPI to default to the number
#9 27.57 of hardware threads instead of the number of processor cores, use the
#9 27.57 --use-hwthread-cpus option.
#9 27.57 
#9 27.57 Alternatively, you can use the --oversubscribe option to ignore the
#9 27.57 number of available slots when deciding the number of processes to
#9 27.57 launch.