Add less strict subsetting functionality

The current subsetting functionality requires that the subregion be entirely in the bounds of the dataset being subset. This can cause a number of problems, especially when dealing with data where the information falls on fractional lat/lon grid values. It would be nice to have a subset function that wouldn't error out if one or more of it's values are outside of the dataset's bounds. Instead, it should return the overlap of the subset bounds and the dataset's bounds.