change default value of grid.ppm.from to 2 (was 5 before)

According to prior agreement with @pavel-shliaha I changed the default value of `grid.ppm.from` to `2` (was `5` before).

The default grid parameter are now:

    ppms <- seq(2, 20, by=2)
    nsds <- seq(0.5, 5, by=0.5)