Fix issue #210, multiple gen per bus error - PQSUM

[todorovski-m]

Before executing the relevant distribution power flow procedures (such as calc_v_pq_sum, calc_v_i_sum, calc_v_y_sum), radial_pf consolidates multiple generators per bus into a single generator. Consequently, the previously erroneous make_zpv function now consistently obtains a vector pv comprising only unique indices. The cumulative reactive power of the combined generator is allocated among the initial generators using pfsoln. Additionally, the test cases in t_pf_radial have been updated to include scenarios where 2 generators per bus are introduced.

Fixes: https://github.com/MATPOWER/matpower/issues/210 Refs: https://www.mail-archive.com/matpower-l@cornell.edu/msg08278.html

[rdzman]

This looks great @todorovski-m! Thanks for the quick turnaround!