Use SVG images for Chapter 7 figures 1 and 2

Makefile

@@ -14,10 +14,10 @@ MAIN_DOC_IMG := NFFFFFF-1.0.png
 MAIN_DOC_IMG += ci_1d_interpolation_subarea.svg ci_2d_interpolation_subarea.svg ci_bounds_interpolation.svg
 MAIN_DOC_IMG += ci_dimensions_overview.svg ci_interpolation_coefficients.svg ci_interpolation_subarea_generation_process.svg
 MAIN_DOC_IMG += ci_quadratic1.svg ci_quadratic2.svg ci_quadratic3.svg mesh_figure.svg
+MAIN_DOC_IMG += order_horizontal_bounds__1D_coord_variables.svg order_horizontal_bounds__2D_coord_variables.svg
 
 #  ... and th list of dynamic files images, with a build rule that appears below
 MAIN_DOC_IMG_BLD := cfdm_cf_concepts.svg cfdm_coordinate_reference.svg cfdm_coordinates.svg cfdm_field.svg
-MAIN_DOC_IMG_BLD += order_horizontal_bounds__1D_coord_variables.png order_horizontal_bounds__2D_coord_variables.png
 
 MAIN_DOC_IMG += $(MAIN_DOC_IMG_BLD)
 
@@ -88,10 +88,4 @@ images/cfdm_coordinates.svg: images/cfdm_coordinates.gv
 	dot -Tsvg $< -o $@
 
 images/cfdm_field.svg: images/cfdm_field.gv
-	dot -Tsvg $< -o $@
-
-images/order_horizontal_bounds__1D_coord_variables.png: images/order_horizontal_bounds__1D_coord_variables.pdf
-	pdftoppm -progress -singlefile -r 300 -png $< $(basename $@)
-
-images/order_horizontal_bounds__2D_coord_variables.png: images/order_horizontal_bounds__2D_coord_variables.pdf
-	pdftoppm -progress -singlefile -r 300 -png $< $(basename $@)
+	dot -Tsvg $< -o $@

ch07.adoc

@@ -89,7 +89,7 @@ For example, if the intervals that contain grid points **`lat(i)`** and **`lat(i
 [caption="Figure {doc-part}.{counter:figure}. ", reftext=Figure {doc-part}.{figure}]
 [.text-center]
 .Order of **`lonbnd(i,0)`** and **`lonbnd(i,1)`** as well as of **`latbnd(i,0)`** and **`latbnd(i,1)`** in the case of one-dimensional horizontal coordinate axes. Tuples **`(lon(i),lat(j))`** represent grid cell centers. The four grid cell vertices are given by **`(lonbnd(i,0),latbnd(j,0))`**, **`(lonbnd(i,1),latbnd(j,0))`**, **`(lonbnd(i,1),latbnd(j,1))`** and **`(lonbnd(i,0),latbnd(j,1))`**.
-image::images/order_horizontal_bounds__1D_coord_variables.png[,50%,pdfwidth=50vw,align="center"]
+image::images/order_horizontal_bounds__1D_coord_variables.svg[,50%,pdfwidth=50vw,align="center"]
 
 Bounds for 2-D coordinate variables with 4-sided cells:: In the case where the horizontal grid is described by two-dimensional auxiliary coordinate variables in latitude **`lat(n,m)`** and longitude **`lon(n,m)`**, and the associated cells are four-sided, then the boundary variables are given in the form **`latbnd(n,m,4)`** and **`lonbnd(n,m,4)`**, where the trailing index runs over the four vertices of the cells.
 Let us call the side of cell **`(j,i)`** facing cell **`(j,i-1)`** the "**`i-1`**" side, the side facing cell **`(j,i+1)`** the "**`i+1`**" side, and similarly for "**`j-1`**" and "**`j+1`**".
@@ -100,7 +100,7 @@ With this notation, the four vertices are indexed as follows: **`0=(j-1,i-1)`**,
 [caption="Figure {doc-part}.{counter:figure}. ", reftext=Figure {doc-part}.{figure}]
 [.text-center]
 .Order of **`lonbnd(j,i,0)`** to **`lonbnd(j,i,3)`** and of **`latbnd(j,i,0)`** and **`latbnd(j,i,3)`** in the case of two-dimensional horizontal coordinate axes. Tuples **`(lon(j,i),lat(j,i))`** represent grid cell centers and tuples **`(lonbnd(j,i,n),latbnd(j,i,n))`** represent the grid cell vertices.
-image::images/order_horizontal_bounds__2D_coord_variables.png[,50%,pdfwidth=50vw,align="center"]
+image::images/order_horizontal_bounds__2D_coord_variables.svg[,50%,pdfwidth=50vw,align="center"]
 +
 If i-j-upward is a right-handed coordinate system (like lon-lat-upward), this ordering means the vertices will be traversed anticlockwise on the lon-lat surface seen from above (<<img-bnd_2d_coords>>).
 If i-j-upward is left-handed, they will be traversed clockwise on the lon-lat surface.