Fix sim driver and simio_timer

[tgtakaoka]

• Fix sim driver so that it can handle two bytes read/write.
• Add error message regarding odd size write to sim driver.
• Fix simio_timer constructor so that can accept size.
• Fix simio_timer so that we read/write channels' registers.

[dlbeer]

Great, thanks for this!

[tgtakaoka]

My pleasure!