Consistency checker

At certain points in the code a consistency check is made, primarily to warn the developers in advance of some error they've made. Errors can also be introduced by careless users, and it would be nice to expose all of these consistency checking functions in some larger routine which sanity-checks the spectral sequence and displays structural problems with it in a notification window. (E.g.: This pair of differentials doesn't compose to zero, these two multiplication specifiers are mutually inconsistent, ... .)