update AEIThorns::Trigger doc, correct parameter steerabiblity, fix skipping non-set triggers

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.

Keyword: Trigger

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