'$' character in '$date' agro format may not be the most appropriate

[marinantoine]

Using the '$' character in a R variable name forces to use quotes around the variable name to access it. May be it could be replaced by another character even if for now the variables are accessed by their number (vec_date = grep("\$date", colnames(d)) in format_data_PPBstats.data_agro.R).

[priviere]

To do it, three places have to be changed:

• R functions
• documentation (inst/bookdown)
• data (all .RData that are called in the documentation with data()