acorn: Fixes for single-density image handling

1. The two sides of a double-sided disk are independent filesystems,
   and both seem to have the per-sector H field set to 0.
2. SSD image files are laid out sequentially if double sided.

Refs #551

Author: keirf

src/greaseweazle/data/diskdefs_acorn.cfg

@@ -99,6 +99,7 @@ disk dfs.ds
         id = 0
         cskew = 3
         rate = 125
+        h = 0
     end
 end
 
@@ -113,5 +114,6 @@ disk dfs.ds80
         id = 0
         cskew = 3
         rate = 125
+        h = 0
     end
 end

src/greaseweazle/image/acorn.py

@@ -9,6 +9,7 @@
 
 class SSD(IMG):
     default_format = 'acorn.dfs.ss'
+    sequential = True
 
 class DSD(IMG):
     default_format = 'acorn.dfs.ds'