[ET Trac] [Einstein Toolkit] #1574: update AEIThorns::Trigger doc, correct parameter steerabiblity, fix skipping non-set triggers

[Einstein Toolkit]

#1574: update AEIThorns::Trigger doc, correct parameter steerabiblity, fix
skipping non-set triggers
-----------------------------------+----------------------------------------
 Reporter:  rhaas                  |       Owner:                     
     Type:  defect                 |      Status:  new                
 Priority:  major                  |   Milestone:                     
Component:  EinsteinToolkit thorn  |     Version:  development version
 Keywords:  Trigger                |  
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 the attached patches do:

 * reduce_int.patch: allow reductions on non-integer variables
 * int_param.patch: allow non-real parameters to be checked
 * doc_scalars.patch: update documentation to mention handling of grid
 scalars
 * no-checkpoint-scalars.patch: don't checkpoint temporary variables
 * ignore_empty_triggers.patch: fix code so that empty paramater/variable
 names cause it to skip a trigger which is what param.ccl documents it to
 do
 * fix_parameter_steerability.patch: some parameters are parsed into a GH
 extension at STARTUP and thus cannot be (meaningfully) steered afterwards.
 This patch marks those parameters as STEERABLE=RECOVER.

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Ticket URL: <https://trac.einsteintoolkit.org/ticket/1574>
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