Compiled workers - Horace

[mducle]

Adds a compiled worker which works with herbert, parpool and mpiexec_mpi type clusters (not verified on slurm_mpi yet but should also work there...). These changes require the equivalent Herbert PR#434 to work.

The changes include removing a check in @hpc_config that a worker must be a Matlab command - if it is not a Matlab function, and it is a valid filename or system command it will be accepted (see the Herbert PR for details).

The compiled worker is not built by default. At the moment to build it you must do:

mkdir build && cd build && cmake .. && cmake --build . --target build_worker

Then specify the built worker in @parallel_config:

ppc = parallel_config; ppc.worker = '<path/to/Horace>/build/admin/compiled_worker.exe';

And run gen_sqw etc to see that it works.