Gen option: Limit recursion to n levels

[nkalivoda]

This is an idea inspired by the theory of recursive feet by Martínez-Paricio and Kager, among various others.

Once we have implemented issue #514, making each minimal φ left- or right-headed, we will be able to fully mimic OTWorkplace's quantity-insensitive stress parse generator, with the following settings:

1. Treat root cat as ω, recursive cat as Ft, and leaf cat as σ.
2. Restrict recursive cat to binary and unary branching.
3. Allow non-exhaustive parsing.
4. Disallow recursion.
5. Require headedness, and it's nGX Gen. Don't require headedness, and it's nGo Gen.

Now, since we already have recursion, it is a short step to dealing with recursive feet as well. There are two ways of thinking about these:

1. Recursion could be unlimited. Think Liberman & Prince (1977). Once issue Swedish accent #514 is done, we'll have this (though only with heads marked at the minimal level, not at all levels like in LP '77).

2. Recursion could be limited, as it is for Martínez-Paricio and others. For MP, the only recursive feet are (σ('σσ)), (σ(σσ')), (('σσ)σ), and ((σ'σ)σ).

To get option to 2, we would use the above options for OTWorkplace-style recursiveless feet, but instead of disallowing recursion, admit exactly one level of recursion. (This doesn't actually address the fact that MP disallows (σ('σ)) and (('σ)σ), so maybe that would have to be another option.)

After writing all of this, I realize that the desired effect might already be nearly available to us depending on the status of Mips and Maps (issue #263).

[nkalivoda]

If we ever implement this, then we should also add a few constraints from Martínez-Paricio & Kager (2015), as well as some more familiar ones like AFL and AFR. But there's no reason to do this before dealing with the Gen option.

[nkalivoda]

We decided just to limit to one level of recursion, not n.