Statistics on ParquetFile subset.

[yohplala]

Fixes #938

[yohplala]

Hi @martindurant
I checked all failing environments, it is systematically related to a trouble with the setting of the conda env. and tests are actually not run.

C:\Users\runneradmin\micromamba-bin\micromamba.exe shell deinit -s bash -p C:\Users\runneradmin\micromamba-root -y --log-level warning
  The following argument was not expected: -p
  Run with --help for more information.
  D:\a\_actions\mamba-org\provision-with-micromamba\main\dist\post\index.js:62[5](https://github.com/dask/fastparquet/actions/runs/11574863592/job/32220088198?pr=940#step:9:5)85
      throw Error(`Failed to execute ${JSON.stringify(args)}: ${error}`)
            ^
  
  Error: Failed to execute ["C:\\Users\\runneradmin\\micromamba-bin\\micromamba",["shell","deinit","-s","bash","-p","C:\\Users\\runneradmin\\micromamba-root","-y","--log-level","warning"]]: Error: The process 'C:\Users\runneradmin\micromamba-bin\micromamba.exe' failed with exit code 10[9](https://github.com/dask/fastparquet/actions/runs/11574863592/job/32220088198?pr=940#step:9:9)

[yohplala]

@martindurant , I am not super expert with the definition of cached properties. Don't hesitate to ask for an alternative fix if this one is not appropriate (I would do with some directions)