This commit introduces the following environment variables that affect
`make test` (or `ctest`) behavior of ALPSCore:

| Variable                      | Default                  | Usual value    | Meaning                                                           |
| `ALPS_TEST_MPIEXEC`           | `${MPIEXEC}`             | `mpiexec`      | MPI launcher                                                      |
| `ALPS_TEST_MPI_NROC_FLAG`     | `${MPIEXEC_NUMPROC_FLAG}`| `-n`           | flag to specify the number of MPI processes                       |
| `ALPS_TEST_MPI_NPROC`         | 1                        | 1              | How many MPI processes to launch in MPI-enabled tests             |
| `ALPS_TEST_MPIEXEC_PREFLAGS`  | `${MPIEXEC_PREFLAGS}`    | (empty string) | MPI launcher arguments preceding the executable name              |
| `ALPS_TEST_MPIEXEC_POSTFLAGS` | `${MPIEXEC_POSTFLAGS}`   | (empty string) | MPI launcher arguments preceding the arguments for the executable |

The `${...}` above are CMake variables, normally set by `find_MPI` module.

Related: issue #211.

This should close #296.

Intended use:

** Case 1: Vanilla MPI-enabled environment. **

The command to run an MPI program using 2 processes: `mpiexec -n 2 some_test`

Setting the variables to run each MPI-enabled tests on 2 processes: `ALPS_TEST_MPI_NPROC=2 make test`

** Case 2: NERSC Cori **

(Disclaimer: not tested with an actual Cori run)

Users are not supposed to run `mpiexec`. One has to allocate interactive nodes first.

Allocating 2 Haswell nodes for 30 minutes: `salloc -N 2 -C haswell -q interactive -t 0:30:00`

Command to run on the allocated nodes: `srun some_test`

Setting the variables to run each MPI-enabled tests on the allocated nodes:
`ALPS_TEST_MPIEXEC=srun ALPS_TEST_MPI_NPROC=' ' ALPS_TEST_MPI_NPROC_FLAG=' ' make test`

(note the variables are assigned spaces, not empty strings!)

** Case 3: Blue Waters **

(Disclaimer: not tested on actual Blue Waters machine)

The `aprun` command is supposed to be used to launch parallel processes from an interactive node
(see https://bluewaters.ncsa.illinois.edu/using-aprun ).

Command to run on 16 cores, using 8 cores per node (that is, 2 nodes), placing the processes on adjacent cores:
`aprun -N 8 -d 1 -n 16 some_test`

Setting the variables to run each MPI-enabled tests with this configuration:
`ALPS_TEST_MPIEXEC=aprun ALPS_TEST_MPI_NPROC=16 ALPS_TEST_MPI_NPROC_FLAG='-N 8 -d 1 -n' make test`

`make test` (or `ctest`) behavior of ALPSCore:

| Variable                    | Default                 | Usual value    | Meaning |
|-----------------------------|-------------------------|----------------|---------|
| `ALPS_TEST_MPIEXEC`           | `${MPIEXEC}`             | `mpiexec`      | MPI launcher                                                      |
| `ALPS_TEST_MPI_NROC_FLAG`     | `${MPIEXEC_NUMPROC_FLAG}`| `-n`           | flag to specify the number of MPI processes                       |
| `ALPS_TEST_MPI_NPROC`         | 1                        | 1              | How many MPI processes to launch in MPI-enabled tests             |
| `ALPS_TEST_MPIEXEC_PREFLAGS`  | `${MPIEXEC_PREFLAGS}`    | (empty string) | MPI launcher arguments preceding the executable name              |
| `ALPS_TEST_MPIEXEC_POSTFLAGS` | `${MPIEXEC_POSTFLAGS}`   | (empty string) | MPI launcher arguments preceding the arguments for the executable |

The `${...}` above are CMake variables, normally set by `FindMPI` module.

Related: issue #211.

This should close #296.

Intended use:

**Case 1: Vanilla MPI-enabled environment.**

The command to run an MPI program using 2 processes: `mpiexec -n 2 some_test`

Setting the variables to run each MPI-enabled tests on 2 processes: `ALPS_TEST_MPI_NPROC=2 make test`

**Case 2: NERSC Cori**

(*Disclaimer:* not tested with an actual Cori run)

Users are not supposed to run `mpiexec`. One has to allocate interactive nodes first.

Allocating 2 Haswell nodes for 30 minutes: `salloc -N 2 -C haswell -q interactive -t 0:30:00`

Command to run on the allocated nodes: `srun some_test`

Setting the variables to run each MPI-enabled tests on the allocated nodes:
`ALPS_TEST_MPIEXEC=srun ALPS_TEST_MPI_NPROC=' ' ALPS_TEST_MPI_NPROC_FLAG=' ' make test`

(note the variables are assigned spaces, not empty strings!)

**Case 3: Blue Waters**

(*Disclaimer:* not tested on actual Blue Waters machine)

The `aprun` command is supposed to be used to launch parallel processes from an interactive node
(see https://bluewaters.ncsa.illinois.edu/using-aprun ).

Command to run on 16 cores, using 8 cores per node (that is, 2 nodes), placing the processes on adjacent cores:
`aprun -N 8 -d 1 -n 16 some_test`

Setting the variables to run each MPI-enabled tests with this configuration:
`ALPS_TEST_MPIEXEC=aprun ALPS_TEST_MPI_NPROC=16 ALPS_TEST_MPI_NPROC_FLAG='-N 8 -d 1 -n' make test`