AppleII support and more features added

[dclawin]

1. Support for AppleII floppies

   Reading GCR formatted floppies is now supported, formattimg and writing is to be done. Images read use phisical sector order. Interleaving is required for use of images in Mame etc.

   Select disk type 6 in monitor ('t 6').

2. Support for DD/SS floppies, e.g. DEC RX50

   Select with 't 5'

3. Monitor command to temporarily modify disk geometry

   'g t,s[,ts]' max tracks, sectors, track spacing (1/2)

4. Monitor command to read low level nibble/flux data

   'n t,d,m' track, delay from index hole/ms, mode

   mode: 1 Read nibble data with no wait 2 Read flux data (cycles between pulses) 3 Read nibbles after delay and waiting for GCR sector header 4 Read nibbles after delay and waiting for GCR data header

5. Caching for disk reads, nibble data

   Buffer size increase to 6656 bytes (1 nibble track) on Atmega MEGA