drop historical data from GLIS dataset

@slgora @ColinKhoury

add this line immediately after reading the GLIS data:

# drop all historical records from GLIS dataset
all_glis_data <- all_glis_data[!(all_glis_data$historical == "y"), ]

Note that for the first batch of 15 crops we will have to deal with this differently, as the data was downloaded in a different format.