introduce proper parsing of math expressions in config files

[doudou]

We should aim at allowing math expressions in config files without resorting to eval(). eval() is horribly dangerous and yukky.

The general scheme would IMO be:

• if the target type is numeric and
• the YAML-parsed type is a string
• then parse the string

Convenience unit operators (5.deg, 22.mm, 1.bar) should be added to the parser.

Ready-to-use math expression parsers are already available as gems. Or, I always wanted to try treetop/parslet to see what they could do.

[jakobs]

sounds good. especially for using things like Math::PI * 0.5, a.s.o.