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. It is recommended to standardise the data before estimation.
See Lin and Shin (2023) for more details.