always name data dimension 'variable' in report functions

[orichters]

• for consistency
• to make sure as.quitte(reportFE(gdx)) et al. successfully finds variables and units because we go into this part of the code

old:

# A tibble: 92,872 × 8
   model     scenario  region variable  unit      period value data
   <fct>     <fct>     <fct>  <fct>     <fct>      <int> <dbl> <fct>
 1 (Missing) (Missing) LAM    (Missing) (Missing)   2005 22.0  FE (EJ/yr)
 2 (Missing) (Missing) OAS    (Missing) (Missing)   2005 27.8  FE (EJ/yr)
 3 (Missing) (Missing) SSA    (Missing) (Missing)   2005 14.5  FE (EJ/yr)
 4 (Missing) (Missing) EUR    (Missing) (Missing)   2005 56.5  FE (EJ/yr)
 5 (Missing) (Missing) NEU    (Missing) (Missing)   2005  5.40 FE (EJ/yr)
 6 (Missing) (Missing) MEA    (Missing) (Missing)   2005 20.5  FE (EJ/yr)
 7 (Missing) (Missing) REF    (Missing) (Missing)   2005 27.2  FE (EJ/yr)
 8 (Missing) (Missing) CAZ    (Missing) (Missing)   2005 11.9  FE (EJ/yr)
 9 (Missing) (Missing) CHA    (Missing) (Missing)   2005 58.6  FE (EJ/yr)
10 (Missing) (Missing) IND    (Missing) (Missing)   2005 14.7  FE (EJ/yr)
# ℹ 92,862 more rows

new:

# A tibble: 92,872 × 7
   model     scenario  region variable unit  period value
   <fct>     <fct>     <fct>  <fct>    <fct>  <int> <dbl>
 1 (Missing) (Missing) LAM    FE       EJ/yr   2005 22.0
 2 (Missing) (Missing) OAS    FE       EJ/yr   2005 27.8
 3 (Missing) (Missing) SSA    FE       EJ/yr   2005 14.5
 4 (Missing) (Missing) EUR    FE       EJ/yr   2005 56.5
 5 (Missing) (Missing) NEU    FE       EJ/yr   2005  5.40
 6 (Missing) (Missing) MEA    FE       EJ/yr   2005 20.5
 7 (Missing) (Missing) REF    FE       EJ/yr   2005 27.2
 8 (Missing) (Missing) CAZ    FE       EJ/yr   2005 11.9
 9 (Missing) (Missing) CHA    FE       EJ/yr   2005 58.6
10 (Missing) (Missing) IND    FE       EJ/yr   2005 14.7
# ℹ 92,862 more rows

[orichters]

Some background info:

• as.quitte interprets d3 or variable as a variable, see this code. I suggested to add data there a while ago, which is what a new.magpie gets as default, but @tscheypidi declined that, fearing that it might break some scripts
• He suggested to set the third to dimension to "variable" for every magpie object in remind2, so this is what I did.