Emergency bug fix for the new roll-out and metadata loader

[yingkaisha]

New feature

• Enable customizable metadata file from conf['predict']
• Can skip metadata if no info is provided
• Can specify a subset of variables to save
• Updated FuXi and crossformer configs based on the new roll-out feature

Bugfix

• Fixed the netCDF4 encoding error of time coordinate.
• Fixed string to float conversion error in the old era5.yml, the original era5.yml is preserved as era5_old.yml.
• Fixed xarray slicing error.
• Fixed metadata loading problem and replaced it with customized metadata location in conf['predict']
• Fixed pbs.py; it will always remove the old launch.sh and model.yml, and copy/generate the new ones in conf['save_loc']. These operations will have print-outs when running rollout_to_netcdf.py and train.py

[yingkaisha]

Proposed a better fix for the time encoding problem, c.f. pydata/xarray#3739