Allow optional thorns in the thorn list

Some thorns are not available everywhere, yet we want to provide a single thorn list for all systems. We discussed the following mechanism:

- There is a way to indicate optional thorns in the thorn list, e.g. via a prefix "!OPTIONAL". These thorns are disabled by default. - There is a way to enable these thorns in the option list. The option list is a good place for this since it is already machine specific. This mechanism should mesh nicely with how options for that thorn are specified. - These specifications in the option list are interpreted by Cactus (not Simfactory), and modify the thorn list before it is interpreted (by Cactus).

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