gp_standard needs deleting

[IhsanKhaliq]

https://github.com/IhsanKhaliq/Ascotracer/blob/087e9d485b495bdf49f0a611277c4dfbbdb62aa6/R/trace_asco.R#L153

There is no gp_standard in the original model, and its value (40) has already been assigned to the min_gp'. P.s. If minimum growing points per day are 350 and maximum growing points per square metre are 15000, then 40 doesn't seem the right value for gp_standard`

[PaulMelloy]

Please don't delete, I renamed this variable in the data.table to consolidate a number of other parameters