Take into account pandas FutureWarning that soon it will not downcast on '.fillna'.

Philipp noticed a pandas FutureWarning regarding deprecation of downcasting on '.fillna':

% exdf-reduce p4462:r166:raw --spi-reduce-cfel --spi-reduce-det-sources "SPB_DET_AGIPD1M-1/DET/0CH0:xtdf" --output-scope none
INFO:exdf.cli.reduce:Opened data from /gpfs/exfel/exp/SPB/202302/p004462/raw/r0166 containing 109 sources and 3109 trains across 257 files
INFO:spi_reduction.exdf_entry.SpiUserSlicing:Using SPIFrameSelectCFEL - reduce SPI data in CFEL format with:
    proposal_num: 4462
    base_path: '/gpfs/exfel/exp/SPB/202302/p004462/scratch/events'
INFO:spi_reduction.exdf_entry.SpiUserSlicing:Selecting data from the detector source SPB_DET_AGIPD1M-1/DET/0CH0:xtdf
/gpfs/exfel/data/user/turkot/dev/spi-data-reduction/src/spi_reduction/user_frames_select.py:94: FutureWarning: Downcasting object dtype arrays on .fillna, .ffill, .bfill is deprecated and will change in a future version. Call result.infer_objects(copy=False) instead. To opt-in to the future behavior, set `pd.set_option('future.no_silent_downcasting', True)`
  ser_xfel_select = ser_user.align(
INFO:exdf.cli.reduce:Applying SpiUserSlicing with 3109 reduction operations
WARNING:exdf.data_reduction.ReduceWriter:Detected EXDF-v1.3 file with attributes mislabeled as EXDF-v1.2
INFO:exdf.cli.reduce:Not writing out any data files