Enhancement/move preprocessing methods

The preprocessing isn't properly shared between data.table and sf methods right now.

Following this SO post, it should be set up like this:

eval_pt <- function(...) {
    # preprocessing steps e.g.
    nm <- deparse(substitute(layer))
    
    UseMethod('eval_pt_', x)
}

eval_pt_ <- function(...) UseMethod("eval_pt_")

eval_pt_.data.table <- function(...) {
    ...
}

eval_pt_.sf <- function(...)
    ...
}

• fix in eval_pt
• fix in eval_buffer
• use new logic in eval_dist
• fix in grid_ct
• update NEWS
• bump version
• tag release
• check tests