Determine uniform strategy for column visibilities of new Keys with two (possibly distinct) parent Keys

[dhutchis]

MultiplyOp in general does not have a clear inheritance for column visibility, because generated keys descend from two parent keys that may have differing visibility.

Some possible strategies:

1. Always set an empty column visibility.
2. Set the column visibility of the parents if they match, otherwise set an empty column visibility.
3. Always set a constant column visibility, specified by the client.
4. Always set the left Key's column visibility. Or always set the right Key's visibility.
5. Set the column visibility to the left Key's c.v. -OR- the right Key's c.v. (take the union of the two). Don't take the union if the c.v.s are the same.
6. Set the column visibility to the left Key's c.v. -AND- the right Key's c.v. (take the intersection of the two). Don't take the intersection if the c.v.s are the same.

I don't know which strategy is the most common use case. I could support all of them with enough options...

Strategy 5 and 6 is possible with the getParseTree() methods. Or we could use simple string operations that add parentheses and the | or & symbol.