R/twa.R: Fix a bug in plotting one-box models of class one_box that affected plots that displayed a time weighted average.
R/PEC_sw_drainage_uk.R: Fix a bug leading to increased PEC values in the case the application date is after the beginning of the drainage period and soil_DT50 was specified.
R/{PEC_sw_drift,PEC_sw_exposit_runoff,PEC_sw_sed}.R: Make use of the units package.
R/PEC_sw_drift.R: Change argument name from 'crop_group_focus' to 'crop_group_RF', in order to make it easier to understand the relation to the 'drift_data' argument.