interaction.c: Introduced interaction_variable and interaction_value

interaction.c: Introduced interaction_variable and interaction_value
structs and associated functions.

covariance-matrix.c: Altered covariance_accumulate_listwise,
covariance_accumulate_pairwise, and update_* functions to handle
interactions.

glm.q: Added temporary NULL and 0 as final args to call to
covariance_matrix_accumulate.