Added the working gridue to bout converter.

src/boutdata/gridue_to_bout_converter/README.md

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+This script was modified from B. Dudson's original gridue_to_bout.py script. It creates a BOUT grid file from a GRIDUE file. 
+
+It can be ran from the terminal.
+
+To run it directly after the creation of a gridue file on INGRID the following code needs to be added to ingrid.py: 
+
+```python
+    def ExportBOUTgrid(self, gridue_file, bout_grid_name: str = 'bout_from_in.grd.nc', plotting = True, verbose = True, ignore_checks = False):
+        """
+        Export a BOUT grid file for the created grid.
+
+        Parameters
+        ----------
+        gridue_file : str, mandatory
+            Name of gridue file to convert to BOUT grid.
+
+        bout_grid_name : str, optional
+            Name of BOUT grid file to save.
+
+        plotting : bool, optional
+            If True, plot the gridue file before conversion.
+
+        verbose : bool, optional
+            If True, print verbose output during conversion.
+
+        ignore_checks : bool, optional
+            If True, ignore checks for gridue file format and structure.
+
+        """
+        bout_grid_name = gridue_file + "_" + bout_grid_name
+        Convert_grids(gridue_file,bout_grid_name,plotting, verbose, ignore_checks)
+
+```
+
+And modifying the `ExportGridue` function to: 
+
+```python
+    if type(self.CurrentTopology) in [SNL]:
+            if self.WriteGridueSNL(self.CurrentTopology.gridue_settings, fname):
+                print(f"# Successfully saved gridue file as '{fname}'")
+                self.ExportBOUTgrid(fname, 'bout_from_in.grd.nc', plotting = True, verbose = True, ignore_checks = False)
+    elif type(self.CurrentTopology) in [SF15, SF45, SF75, SF105, SF135, SF165, UDN, CDN]:
+        if self.WriteGridueDNL(self.CurrentTopology.gridue_settings, fname):
+            print(f"# Successfully saved gridue file as '{fname}'")
+            self.ExportBOUTgrid(fname, 'bout_from_in.grd.nc', plotting = True, verbose = True, ignore_checks = False)
+```
+
+*Note:* The created file will have 
+`dimensions:
+        x ;
+        y ;
+        z ;
+        x2 ;
+        y2 ;
+        t = UNLIMITED ; //`
+
+Where x has guard cells following INGRID's convention, and y doesn't have any. The converter takes the BOUT coordinates to be x -> y2 (and removes the guard cells) and y -> x2 (and adds the guard cells). So BOUT ends up using x2 as X and y2 as Y coordinates.