API: Unify usage of N parameter for tomography methods.
API: Unify usage of N parameter for tomography methods.

Previously, the N parameter could either refer to the number of
distirbutions, the range of scans to process, or the index of the
last scan to process. Now N only refers to the range of scans to
process, and NUM_DIST is the parameter for distributions.

carterbox/wpart