include hereditary constraints

[merliseclyde]

Add option for factors and interactions so that

1. all levels of a factor enter or leave the model together
2. all lower order terms are included whenever a higher order term is added to the model for i.e. for a model formula with factor A having J levels and continuous variable B: Y ~ A*B
   there are 4 models Int, Int + A, Int + B, Int + A + B + A:B
   Should work with higher order interactions in factorial models.

[merliseclyde]

Works with bas.lm and bas.glm with methods BAS and MCMC; currently does not work with method deterministic or method BAS+MCMC. Not compatible with include.always yet.