Get points and/or resolution only if needed

[mhesselbarth]

Currently, when running calculate_lsm() and window_lsm() we get the coordinates of all cells using raster_to_points and the resolution no matter if we need them for the selected metrics. We do this before actually calculating the metrics because this allows us to get them only once and not for each metric. However, maybe it makes sense to first check what we actually need.

[mhesselbarth]

Related to #154