BUG: The npv function in financial.py was incorrectly implemented.

Correct the implementation of the npv function, its documentation, and
the mirr function that depends on it. The test_financial.py is also
corrected to take into account those modifications

The npv function behavior was contrary to what the documentation stated
as it summed indexes 1 to M instead of 0 to M-1. The mirr function used
a corrective factor to get the correct result in spite of that error so
that factor is removed.

Closes #649