Changed how linear index enters class Basic

Linear index removed from score_input function. Instead, added xb (the linear index except for the
constant term) to the score function. This will help later when using the score_input function in
the class FMM.
Simplified model substantially

The model now is:

y = xb + zg + (a + e)

where a is drawn from (-mu, 0, mu)

with symmetric distribution.

In words: there are three types; the middle type is zero and the left type is -mu while the right
type is mu. Left and right types have equal probability.