adafruit_sdcard.py : group related cmds in a single transaction

Most cards tested wants to have CS low during a group of read-multiple / write-multiple commands.
Also, some can insert 0x7F return bytes after CMD12 (response type R1b)

These changes make reading and writing big chunks of data at once possible, on all cards tested, where previously all but the oldest one would fail when reading/writng more than a block.

Author: Anton-2