Add --startSingleProcess flag to submit and exec sub-commands

[sverhoeven]

The Xenon JobDescription has a setStartSingleProcess() method which is not exposed to xenon-cli at the moment. This flag should be added so threaded jobs can be submitted.

[sverhoeven]

Tested against slurm with

docker run --detach --publish 10022:22 nlesc/xenon-slurm
./gradlew installDist

./build/install/xenon/bin/xenon scheduler slurm \
 --prop xenon.adaptors.schedulers.ssh.strictHostKeyChecking=false \
--location ssh://localhost:10022 --username xenon --password javagat \
submit --stdout test-%j.out --procs-per-node 2 hostname
# Has the hostname twice in test-*.out

./build/install/xenon/bin/xenon scheduler slurm \
--prop xenon.adaptors.schedulers.ssh.strictHostKeyChecking=false \
--location ssh://localhost:10022 --username xenon --password javagat \
submit --stdout test-%j.out --procs-per-node 2 --start-single-process hostname
# Has the hostname once in test-*.out