Expressing Constraints in Instrument and Science Model Packs

[jdtsmith]

We need a simple and flexible means of expressing constraints on and between parameters, for the input specification discussed in #9.

Examples that can be attached to one parameter:

1. Parameter is fixed: fixed (or omit). Example: temperature of starlight continuum.
2. Parameter is bounded (below, above, or both): bounded: [p1,] or bounded: [,p2], or bounded: [p1,p2]. Examples: central intensities, wavelengths, etc.
3. Parameter is tied to another parameter (by name?): tied: 1.15 pah_77_feature1_central_lam. Example: fixing ratios between (probably new) sub-features of PAH complexes.

Examples that might need to be expressed separately, since they govern multiple parameters:

1. Parameter is tied to a simple function of another parameter: tied: integrated_strength(pah_11*) = 0.4 integrated_strength(pah_77*) or some such.

Instrument packs can use this notation as well.