Drives: head stepping should take time, heads should have width and an inner stop

[TomHarte]

On the Apple II:

• DiskII head step takes time to move (~1ms per 1/4 track for half-height drives, full-height is ~1.5ms);
• DiskII inner head stop is at track 36.5;
• Write/erase head width is ~0.8 track, so writing a track also writes to adjacent 1/4 tracks.

On other machines, stepping and settling time should also be something other than zero, and ~0.8 tracks is not necessarily a bad choice for head width. Most disk image formats don't support sub-track placement, so it largely won't be noticeable. Find out if possible what the size is on the C1540.