Using the sequence_space converter to allow for multiple grid inputs into `grdimage`. Useful when users want to make an RGB image plot e.g. from Landsat images (Bands 4,3,2).

Extends `grdimage` to accept a list of individual red, green and blue xarray.DataArrays for plotting. Had to revert using the 'sequence_space' decorator as it would not work nicely for multiple grids... Implementation uses the contextlib.ExitStack method to create multiple virtual grid files and enter multiple contexts (similar to 3448972). Also added one test case with dummy data, but could do with more to capture edge cases.