Reinstate figure directory for documentation

The directory must be specified with full path to refer to the source
directory. The images will not be referred to by this path, but rather
copied to the output directory (as of Doxygen 1.7.6.1 at least), unlike
the stylesheet (consistency is so last decade).

.gitignore

@@ -32,7 +32,7 @@ Makefile
 bin/
 lib/
 Doxyfile
-Documentation
+Documentation/html
 dune.module
 *.pc
 install_manifest.txt

Documentation/Figure/.gitignore

@@ -0,0 +1 @@
+# intentionally empty -- present just to get directory created

Doxyfile.in

@@ -688,7 +688,7 @@ EXAMPLE_RECURSIVE      = NO
 # directories that contain image that are included in the documentation (see
 # the \image command).
 
-IMAGE_PATH             =
+IMAGE_PATH             = @PROJECT_SOURCE_DIR@/Documentation/Figure
 
 # The INPUT_FILTER tag can be used to specify a program that doxygen should
 # invoke to filter for each input file. Doxygen will invoke the filter program