Consider removing SigmetTacElement

[kasarit]

I find this class a bit strange. It seems that it is never used in parsing, but only upon reconstruction. But we already have lexemes for TAC geometry that we could use for reconstruction, right? I'd suggest removing this class and use the lexeme reconstructors we already have for this purpose.

But if there's a good reason to retain this class, I'd propose to at least

• make the outer class empty, without extending or implementing any class (and omit it in any configuration),
• rename the outer class and lexeme identity related to TAC geometry, and
• deduplicate handing of SIGMET and AIRMET.

Originally posted by @petringo in #117 (comment)