better handle soil water potential inputs

[RamiALBASHA]

The user can currently use 'psi_soil' arg in model.run() to force a constant soil water content across the simulation period. Alternatively when 'psi_soil' is not provided, the user is expected to provide 'psi_soil.input' which provides soil water potential values that are ONLY used at midnight.

• BUG: if hour == 0 and soil water potential is not provided for the current day, soil water potential will not be calculated (code)
• improvement: add 'psi_soil_init' as an input arg. If done, modify the verification control (here) so that error will be raised only when neither 'psi_soil_init' not 'psi_soil.csv' are provided