Does log work in curve3d?

I expected the following code to produce a plot with logged axes, but instead it produces a picture that has the same contours as without the log command (the labels are moved, though!)

``` R
library(emdbook)

ran = c(0.25, 2)

KW <- function(x, y){
return(x^2+y^2)
}

cc <- curve3d(KW(x, y)
, xlim = ran
, ylim = ran
, n = 51
, sys3d = "contour"
, log = "xy"
)
```

It produces the following warnings

``` Rlog
Warning messages:
1: In seq.default(log(from[1]), log(to[1]), length = n) :
first element used of 'length.out' argument
2: In seq.default(log(from[2]), log(to[2]), length = n) :
first element used of 'length.out' argument
```

Start by reproducing the supplied R example at the curve3d entry point and inspect how its log argument is handled when generating x and y sequences. Done means the log="xy" example produces contours using logged axes without the reported seq.default warnings.

Written by the indexing model from the issue text.