Enhance BEAGLE options

[rbouckaert]

BEAGLE has gained quite a few more options, but controlling these is currently not supported.
In particular, it can benchmark various BEAGLE instances (e.g. CPU vs GPU) and choose the fastest available.
There are also a number of scale strategy options, e.g. interval of rescaling when dynamic scaling is used.
If both CUDA and OpenCL instances are available, choosing between them requires selecting the right instance number for the beagle-order option, but there is no easy way to select it (like -beagle_SSE enabling SSE).

Instead of adding separate options for each of these to BEAST, adding a single -beagle_options followed by a comma separated list of key-value pairs would give more flexibility in adding more options in the future, e.g.
beast -beagle_options instance=CUDA,precision=single,scaling=dynamic beast.xml