create_y_matrix: Create Y Matrix
In kbrevoort/tidysynth: What the package does (short line)

This function creats a data.frame that contains the information on the time- varying variables.

1	create_y_matrix(data, i_name, t_name, y)

`data`	A data.frame
`y`	A character variable with the name of any y variables to include
`i_var`	A character variable with the name of the unit variable (i)
`t_var`	A character variable with the name of the time variable (t)
`g_def`	A character variable with the boolean logic to identify observations in the treatment group.
`time_span`	An optional vector with up to three values: The earliest value of t_var allowed, the latest value that is part of the pre-treatment period, and the latest time period allowd.