provenance of recipe_combined_indices.yml broken

[bouweandela]

When I run recipe_combined_indices.yml, it results in the following error:

  File "/pf/b/b381141/src/esmvalgroup/esmvalcore/esmvalcore/_task.py", line 583, in _collect_provenance
    if ancestor_file in ancestor_products:
TypeError: unhashable type: 'list'

This is because the diagnostic write the provenance file run/combine_indices/main/diagnostic_provenance.yml like this:

? /pf/b/b381141/failed/recipe_combined_indices.yml/recipe_combined_indices_20200709_094241/work/combine_indices/main/tos__Dec-Feb_MPI-ESM-MRbcc-csm1-119501951_.nc
: ancestors:
  - - /pf/b/b381141/failed/recipe_combined_indices.yml/recipe_combined_indices_20200709_094241/preproc/combine_indices/tos/CMIP5_MPI-ESM-MR_Omon_historical_r1i1p1_tos_1950-1951.nc
    - /pf/b/b381141/failed/recipe_combined_indices.yml/recipe_combined_indices_20200709_094241/preproc/combine_indices/tos/CMIP5_bcc-csm1-1_Omon_historical_r1i1p1_tos_1950-1951.nc
  authors:
  - perez-zanon_nuria
  - hunter_alasdair
  - manubens_nicolau
  projects:
  - c3s-magic
  caption: Combined selection
  statistics:
  - other
  realms:
  - atmos
  themes:
  - phys

while it should look like this:

? /pf/b/b381141/failed/recipe_combined_indices.yml/recipe_combined_indices_20200709_094241/work/combine_indices/main/tos__Dec-Feb_MPI-ESM-MRbcc-csm1-119501951_.nc
: ancestors:
  - /pf/b/b381141/failed/recipe_combined_indices.yml/recipe_combined_indices_20200709_094241/preproc/combine_indices/tos/CMIP5_MPI-ESM-MR_Omon_historical_r1i1p1_tos_1950-1951.nc
  - /pf/b/b381141/failed/recipe_combined_indices.yml/recipe_combined_indices_20200709_094241/preproc/combine_indices/tos/CMIP5_bcc-csm1-1_Omon_historical_r1i1p1_tos_1950-1951.nc
  authors:
  - perez-zanon_nuria
  - hunter_alasdair
  - manubens_nicolau
  projects:
  - c3s-magic
  caption: Combined selection
  statistics:
  - other
  realms:
  - atmos
  themes:
  - phys