Need to separate externals for CPU and GPU

[batatasfritas]

We need a way to differentiate externals that work on different types of hardware.

When I log in to Izar I see (at least) an external that does not run on GPU and I'm about to add an external that runs only on GPU but that will be available on other clusters as well.

Perhaps in the external_packages we need boolean keywords for:

• runs_on_cpu
• runs_on_gpu

So that we can end up with three variations of externals:

• codes that work on CPU but may benefit of GPU acceleration (e.g. Gaussian);
• codes that run on CPU only (likely the majority);
• codes that depend on GPUs and don't have a CPU-only counterpart (e.g. TeraChem)

This would hopefully interact with platforms/<version>/<cluster>.yaml and take an action based on the gpu filter.