Harmonize filter signatures

[danielhuppmann]

This issue provides background on an inconsistency (in my opinion) of the current "nestedness" of filters.

model/scenario
  name, id - columns of this table

unit
  name - a column of this table  
  [id - a column of this table not allowed as filter]

region
  name - a column of this table  
  [id - a column of this table not allowed as filter]
  
run
  default_only - a function of `is_default`
  is_default, id, version - columns of this table
  model -> filters of `model`
  scenario -> filters of `scenario`
  iamc
    variable -> filters of `variable`
    unit -> filters of `unit`
  [meta - not implemented]

iamc.variable
  name - a column of this table  
  [id - a column of this table not allowed as filter]

iamc (= iamc.datapoints & iamc.timeseries & iamc.measurand)
  run  
    default_only
    is_default, id, version - columns of this table
    [model & scenario filter not allowed as filter, compare `runs`]
    [meta - not implemented]
  model -> filters of `model`
  scenario -> filters of `scenario`
  variable -> filters of `variable`
  unit -> filters of `unit`
  
meta
  run  
    default_only
    is_default, id, version - columns of this table
    model -> filters of `model`
    scenario  -> filters of `scenario`
    [iamc - not implemented]
  key - a column of this table
  [value - a column of this table not allowed as filter]

My main concern is that model and scenario are top-level filters in the iamc repository but are nested within the "run"-filter for the meta repository.