don't warn about most nesting

[telic]

The NestingValidator doesn't need to be quite so picky. Most tags should be allowed to cross without warning or error, including self-nested tags.

There are a few exceptions:

• sectioning elements (FRONTMATTER, BACKMATTER, DIV, ACT, SCENE) should not cross MARG or BRACEGROUP
• ACT, SCENE, and DIV should not cross others of the same type
• DIV should not cross FRONTMATTER or BACKMATTER
• ORNAMENT should not cross lines
• CL and LD should not cross each other
• COL should not cross itself

All those mentioned above should be errors, not simply warnings.

[ubermichael]

If A can't split B, then can B split A?

This is a split warning:

<WORK><A><B></A></B></WORK>

Is this?

<WORK><B><A></B></A></WORK>

[telic]

I can't think of any case where it wouldn't be symmetric. A splits B at the same time that B splits A.

[ubermichael]

Add a schema attribute on Tag, noSplit="HW, S, FOO" etc.

[telic]

Missing from the list above: SIG, RT, PN, and CW should never cross one another either.