[ET Trac] [Einstein Toolkit] #876: Allow optional thorns in the thorn list

[Einstein Toolkit]

#876: Allow optional thorns in the thorn list
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 Reporter:  eschnett     |       Owner:     
     Type:  enhancement  |      Status:  new
 Priority:  major        |   Milestone:     
Component:  Cactus       |     Version:     
 Keywords:               |  
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 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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Ticket URL: <https://trac.einsteintoolkit.org/ticket/876>
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