The user-supplied data.frame should contain at least four columns, namely the
dependent variable (\(y_{ijt}\)), block identifier (\(i\)), individual
identifier (\(j\)), and time (\(t\)). The user needs to supply their corresponding
headers in the data.frame to the function using the parameters "depvar_header",
"i_header", "j_header", and "t_header", respectively. If the data is supplied
as a list, these arguments will not be used.
If either r0 = NULL or ri = NULL, both of them will be estimated.
In such case, "r_max" must be supplied. If "r0" and "ri" are supplied then
"r_max" is not needed and will be ignored.
If standarise = TRUE, each time series will be standardised so it has zero mean
and unit variance.