More flexible expression syntax

[Starstrider42]

Currently, most orbital parameters in Custom Asteroid configs must be either floating-point literals, a Ratio expression, or a Offset expression. The latter two are both counterintuitive and inflexible; it would be more useful to replace them with syntax similar to everyday mathematical expressions.

The new syntax should allow numerical parameters to be an Expression, defined as one of the following:

• a floating point literal
• <planet>.<value>
• Resonance(<planet>, <m>:<n>)
• (Expression)
• -Expression
• Expression [+|-|*|/] Expression
  The syntax will not only be easier to work with, but will lend itself easily to future extensions.