using the dsn and a dummy placeholder grid

[mdsumner]

not ready for merging yet, I'll work in this branch and see if stuff works while I think about refactoring for items below

gdalio_get_default_grid() gets dsn argument, and essentially sets the default grid from your source

• [ ] probably we don't want get and set, we can use an op <- options(opt = a) model ?
• [ ] it's a bit weird that get can set, so see previous item
• [ ] ideally we want helpers like extend(+/-) and refine(+/-) for extent and dimension to modify the current grid in relative terms ...