Provenance is not recorded for fx files used by preprocessors

[bouweandela]

Describe the bug
The provenance of fx files used by preprocessors that need fx files, e.g. area_statistics, is not recorded.

Please attach

• The recipe that you are trying to run, you can find a copy in the run directory in the output directory
  recipe_python.yml.txt
• The main_log_debug.txt file, this can also be found in the run directory in the output directory
  main_log_debug.txt

Resulting provenance
tas_amsterdam_CMIP5_CanESM2_Amon_historical_r1i1p1_tas_1850-2000_provenance.svg.txt
tas_amsterdam_CMIP5_CanESM2_Amon_historical_r1i1p1_tas_1850-2000_provenance.xml.txt

(files renamed to .txt so GitHub allows attaching them)