CHIRPS implementation into Pycast

Historical reference data should be changed from ERA5-Land to CHIRPS.

• Current location of CHIRPS at 0.1 resolution: '/pd/data/regclim_data/gridded_data/precipitation/CHIRPS/daily/0.1'
• Available time period: [1981 - 2018]
• File names: 'chirps-v2.0.1981.days_p_0.1_con.nc'

Adaption in Code:

1. domain_config -> "reference_history"
2. Hard coded prefix in truncate_reference
3. Variable name in CHIRPS is "precip": variable_config