ERA5 data inputs

Request implementation to allow use with ERA5 data in rt52:
- hourly screen temperature data in `/g/data/rt52/era5/single-levels/reanalysis/2t/$year/2t_*.nc` (variable name is `t2m` in units of K)
- hourly total precipitation data in `/g/data/rt52/era5/single-levels/reanalysis/mtpr/$year/mtpr_*.nc` (variable name is `mtpr` in kg m**-2 s**-1units of )

I assume icclim does not handle hourly dat, so an additional argument can be passed to run_icclim.py, which tells xr.dataarray how to preprocess the hourly data first such as resample('1D').mean() etc.