Improved timescale parsing

[christian-krieg]

• Now using decimals to avoid rounding errors
• Renamed timescale attribute from "number" to "magnitude"
• Added checks for standard-compliant magnitude values

[cirosantilli]

Thanks, I merged this as: https://github.com/cirosantilli/vcdvcd/commit/cf082797dc09071400812e61d265b7d5dcd948f7

I changed the interface slightly together with a larger refactoring BTW, should be easy to see what's going on.