Master list: improvements
## `grid_ct`
* [x] return IDs: focal ID, grid ID (#16)
* [ ] warn that attributes are rep'ed (e.g.: density may be high for focal, but listed as high for all grid points, regardless of distance)
## `select_ct`
* [x] drop NAs in / at least definitely out
* [x] what about sub = dem > 250? (#15)
* [ ] if x is result of `make_grid` -> `eval_`, should `select_` do summary stats? (#17)
* [ ] auto import of `units::as_unit` for `select_ct` sub, etc ?
## `binary_layer`
* [ ] faster? no double `mask`?
## `eval_*`
* [x] fix sharing of preprocessing between methods (!15)
* [ ] how to summarize categorical and ordinal?
* [ ] test attributes
## `eval_dist`
* [ ] check types of x and layer
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