Transition from raster to terra

[Rafnuss]

terra is the new package replacing raster: https://github.com/rspatial/terra

terra:rast cannot be saved: rspatial/terra#549, we we will convert it to matrix.
Also, metadata has been removed in terra, so we use a new structure to store the data.

[
    {
        stap = 1,
        start = POSIXct(), 
        end = POSIXct(),
        likelihood = matrix(),
        extent = c(xmin, xmax, ymin, ymax)
    }, 
...
]