Added additional checking to ttest formulas

Checking to prevent analysis with IVs containing more than 2 levels.
added callback for meta.t sampling
callbacks called from meta.ttestBF
removed (inappropriate) callback from meta.ttestBF
meta.ttestBF callbacks operational
added support for callbacks in some C functions

(callbacks are still inoperable, however)
added callback for meta.t sampling
removed (inappropriate) callback from meta.ttestBF
working callbacks for nWayGibbs, testGibbs
Merge branch 'JASPcallbacks' of https://github.com/richarddmorey/BayesFactor into JASPcallbacks