Load mrsea survey data from the MRSea package and convert to spatial formats defined by the sp package.
Requires the MRSea package from https://github.com/lindesaysh/MRSea, and
is normally only run by the package maintainer. For regular inlabru use of the data,
use data("MRSea", package = "inlabru")
, which does not require the MRSea package.