Add a regional gridded magnetic dataset

[leouieda]

This will be needed for all of the grid-related processing functions. It would be troublesome to have to generate the grid every time.

This could either be a new dataset from an existing grid or we can grid something ourselves and use it. Not sure what's best.

[leouieda]

@LL-Geo suggested this: http://dx.doi.org/doi:10.26179/5e015bb8dce7f