Update to use `terra` rather than `raster` package

[jflowernet]

Hi,
Firstly, many thanks for the ipdw packge. It is very useful for those of us wanting to do interpolation in the marine realm!

It would be great if the code could be updated to use the terra package rather than raster. Although raster still exists, the package author recommends using terra now, and it is likely the equivalent functions in terra will be faster.

[jsta]

Thanks for your interest. I am happy to accept PRs to this effect or transfer ownership to someone willing to take ownership of this package.

[jflowernet]

Don't know if I would want to take over ownership since I haven't done any work on the package, but happy to do a PR with a terra code update. My intention would be to replace all raster functions with terra equivalents, so ipdw would no longer work with raster inputs. Is that ok?
May be a few weeks before I can get it done, not that I think there is any rush!

[jsta]

Sounds good to me